Encontrei os seguintes recursos que podem ter o que você está procurando.
- Criando um teclado XKB
- Guia não confiável para a configuração do XKB
- A extensão do teclado X: Especificação de protocolo - X Consortium Standard
- Extensão XKB - X Keyboard
Acho que começaria com o quarto link para o site X.org. Houve também um comentário lá:
The XKB data files for the various keyboard models, layouts, and locales are now maintained by the X Keyboard Config project on freedesktop.org.
Assim, o link para freedesktop.org pode ser o melhor lugar para começar também. Especificamente, eles descrevem a missão como tal:
There are many X Window implementations which have very poor support for non-standard keyboards, national layouts and options.
Open Source X Window System implementations (xfree86, x.org) currently have non-synchronized XKB configuration databases: the bugs fixed in one repository are not reflected in the other, new configuration elements are added (in best case) separately and independently to both CVS trees - but usually only one tree gets them. Also, these implementations contain unbalanced and unstructured layout trees (very often one country/language have several layouts, each with its own set of variants).
Commercial X Window System implementations cannot support large variety of national layouts - usually, because of lack of resources. So users from "exotic" countries feel offended and frustrated - their environments are not complete.
The solution which would guarantee quality support for the keyboard configuration data is to have single repository which would serve as meta-project for X servers and OS distributions. X Keyboard Configuration Database is trying to be the one.
Freedesktop.org também mantém uma lista de discussão que pode ser melhor para ajudar você a navegar pelo mar de informações sobre o XKB.