I completed the stage that takes keyboard layout files from XKB (X.Org) and converts them to XML documents, based on a keyboard layout Relax NG schema. Then, these XML documents can also be converted back to keyboard layout files.

Here is an imaginary example of a keyboard layout file.

// Keyboard layout for the Zzurope country (code: zz).
// Yeah.

partial alphanumeric_keys alternate_group hidden
xkb_symbols "bare" {
   key <AE01> { [        1, exclam,      onesuperior,  exclamdown      ] };
};

partial alphanumeric_keys alternate_group
xkb_symbols "basic" {
   name[Group1] = "ZZurope";

   include "zz(bare)"

   key <AD04> { [        r, R,           ediaeresis,   Ediaeresis      ] };
   key <AC07> { [        j, J,           idiaeresis,   Idiaeresis      ] };
   key <AB02> { [        x, X,           oe,           OE              ] };
   key <AB04> { [        v, V,           registered,   registered      ] };
};

partial alphanumeric_keys alternate_group
xkb_symbols "extended" {
    include "zz(basic)"
    name[Group1] = "ZZurope Extended";
    key.type = "THREE_LEVEL"; // We use three levels.
    override key <AD01> {   type[Group1] = "SEPARATE_CAPS_AND_SHIFT_ALPHABETIC",

[ U1C9, U1C8], [  any,   U1C7 ]   }; // q
    override key <AD02> {   [ U1CC, U1CB, any,U1CA ],

type[Group1] = "SEPARATE_CAPS_AND_SHIFT_ALPHABETIC" }; // w

    key <BKSP> {

        type[Group1]="CTRL+ALT",

        symbols[Group1]= [ BackSpace,   Terminate_Server ]

    };

    key <BKSR> { virtualMods = AltGr, [ 1, 2 ] };

    modifier_map Control { Control_L };

    modifier_map Mod5   { <LVL3>, <MDSW> };

    key <BKST> { [1, 2,3, 4] };

};

When converted to an XML document, it looks like

<?xml version="1.0" encoding="UTF-8"?>
<layout layoutname="zz">
  <symbols>
    <mapoption>hidden</mapoption>
    <mapoption>xkb_symbols</mapoption>
    <mapname>bare</mapname>
    <mapmaterial>
      <tokenkey override="False">
        <keycodename>AE01</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>1</symbol>
            <symbol>exclam</symbol>
            <symbol>onesuperior</symbol>
            <symbol>exclamdown</symbol>
          </symbolsgroup>
        </keysymgroup>
      </tokenkey>
    </mapmaterial>
  </symbols>
  <symbols>
    <mapoption>xkb_symbols</mapoption>
    <mapname>basic</mapname>
    <mapmaterial>
      <tokenname name="ZZurope"/>
      <tokeninclude>zz(bare)</tokeninclude>
      <tokenkey override="False">
        <keycodename>AD04</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>r</symbol>
            <symbol>R</symbol>
            <symbol>ediaeresis</symbol>
            <symbol>Ediaeresis</symbol>
          </symbolsgroup>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="False">
        <keycodename>AC07</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>j</symbol>
            <symbol>J</symbol>
            <symbol>idiaeresis</symbol>
            <symbol>Idiaeresis</symbol>
          </symbolsgroup>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="False">
        <keycodename>AB02</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>x</symbol>
            <symbol>X</symbol>
            <symbol>oe</symbol>
            <symbol>OE</symbol>
          </symbolsgroup>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="False">
        <keycodename>AB04</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>v</symbol>
            <symbol>V</symbol>
            <symbol>registered</symbol>
            <symbol>registered</symbol>
          </symbolsgroup>
        </keysymgroup>
      </tokenkey>
    </mapmaterial>
  </symbols>
  <symbols>
    <mapoption>xkb_symbols</mapoption>
    <mapname>extended</mapname>
    <mapmaterial>
      <tokenname name="ZZurope Extended"/>
      <tokeninclude>zz(basic)</tokeninclude>
      <tokentype>THREE_LEVEL</tokentype>
      <tokenmodifiermap state="Control">
        <keycode value="Control_L"/>
      </tokenmodifiermap>
      <tokenmodifiermap state="Mod5">
        <keycodex value="LVL3"/>
        <keycodex value="MDSW"/>
      </tokenmodifiermap>
      <tokenkey override="True">
        <keycodename>AD01</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>U1C9</symbol>
            <symbol>U1C8</symbol>
          </symbolsgroup>
          <symbolsgroup>
            <symbol>any</symbol>
            <symbol>U1C7</symbol>
          </symbolsgroup>
          <typegroup value="SEPARATE_CAPS_AND_SHIFT_ALPHABETIC"/>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="True">
        <keycodename>AD02</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>U1CC</symbol>
            <symbol>U1CB</symbol>
            <symbol>any</symbol>
            <symbol>U1CA</symbol>
          </symbolsgroup>
          <typegroup value="SEPARATE_CAPS_AND_SHIFT_ALPHABETIC"/>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="False">
        <keycodename>BKSP</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>BackSpace</symbol>
            <symbol>Terminate_Server</symbol>
          </symbolsgroup>
          <typegroup value="CTRL+ALT"/>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="False">
        <keycodename>BKSR</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>1</symbol>
            <symbol>2</symbol>
          </symbolsgroup>
          <tokenvirtualmodifiers value="AltGr"/>
        </keysymgroup>
      </tokenkey>
      <tokenkey override="False">
        <keycodename>BKST</keycodename>
        <keysymgroup>
          <symbolsgroup>
            <symbol>1</symbol>
            <symbol>2</symbol>
            <symbol>3</symbol>
            <symbol>4</symbol>
          </symbolsgroup>
        </keysymgroup>
      </tokenkey>
    </mapmaterial>
  </symbols>
</layout>

When we convert the XML document back to the XKB format, it looks like

hidden xkb_symbols "bare"
{
	key <AE01> { [ 1, exclam, onesuperior, exclamdown ] };
};

xkb_symbols "basic"
{
	name = "ZZurope";
	include "zz(bare)"
	key <AD04> { [ r, R, ediaeresis, Ediaeresis ] };
	key <AC07> { [ j, J, idiaeresis, Idiaeresis ] };
	key <AB02> { [ x, X, oe, OE ] };
	key <AB04> { [ v, V, registered, registered ] };
};

xkb_symbols "extended"
{
	name = "ZZurope Extended";
	include "zz(basic)"
	key.type = "THREE_LEVEL";
	modifier_map Control { Control_L };
	modifier_map Mod5 { <LVL3>, <MDSW> };
	override key <AD01> { [ U1C9, U1C8 ], [ any, U1C7 ], type = "SEPARATE_CAPS_AND_SHIFT_ALPHABETIC"  };
	override key <AD02> { [ U1CC, U1CB, any, U1CA ], type = "SEPARATE_CAPS_AND_SHIFT_ALPHABETIC"  };
	key <BKSP> { [ BackSpace, Terminate_Server ], type = "CTRL+ALT"  };
	key <BKSR> { [ 1, 2 ], virtualMods = AltGr  };
	key <BKST> { [ 1, 2, 3, 4 ] };
};

Some things are missing such as partial, alphanumeric_keys and alternate_group, which I discussed with Sergey and he said they should be ok to go away.

In addition, we simplify by keeping just Group1 (we do not specify it, as it is implied).

I performed the round-trip with all layout files, and all parsed and validated OK (there is some extra work with the level3 file remaining, though).

Some issues that are remaining, include

• Figuring out how to use XLink to link to documents in the same folder (+providing a parameter; the name of the variant), and how to represent that in the Relax NG schema.
• Sort the layout entries by keycode value.