I don't know if this is possible from xkb-config; maybe it can be bumped upstream if it is feasible... Recently upgrading to Ubuntu Hardy, I lost the bespoke xkb config I had written for myself. Nothing too special, it had paleographic and typographic keys set up, typographic quotes, a few archaic characters for writing Old English, Old Irish, that sort of thing. But after the upgrade I must now either be content with manually running xkbconfig to load a standalone config (and have it unavailable to the KDE (or Gnome, for that matter) keyboard tools, or go in and hack the main config files to add it back. Again. It would be nice if there was a way of including userspace configs in the main config area: so that you could put your own layouts in, say, ~/.xkb, and have them available from the usual desktop xkb managers. This might require hacking on the various managers themselves, rather than through xkb-config or the xkb system, but I'm fairly sure those people are reading these bugs as well.
"XKB itself" (setxkbmap and xkbcomp utilitu) provide -I option which allows using whatever XKB data you have. It is an issue with GNOME/KDE that do not allow it. As a maintainer of GNOME keyboard configuration, I must admit it is a missing feature, right. You can file a bug in gnome bugzilla (feature request, actually). No promises though - it is not trivial to make user-level xkb configuration db (and make it usable, easy to create and maintain etc etc).
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