Jani, I don't have the /etc/X11/xkb/rules/base.xml file in Gentoo, it probably is part of the old X.Org xkbdata data (the Gentoo package of x11-misc/xkbdata conflicts with the installed x11-misc/xkeyboard-config package). The Debian installer and Ubiquity should use the new xkeyboard-config data.
Jani, I don't have the /etc/X11/ xkb/rules/ base.xml file in Gentoo, it probably is part of the old X.Org xkbdata data (the Gentoo package of x11-misc/xkbdata conflicts with the installed x11-misc/ xkeyboard- config package). The Debian installer and Ubiquity should use the new xkeyboard-config data.