This is definitely desirable. For the current status, see the comment here:

I provide some additional details here on the problem, in case someone other than me reads this and wants to jump in.

First, the format of the "rules" file is described here: https://github.com/xkbcommon/libxkbcommon/blob/master/doc/rules-format.txt

Currently, the most used file is the "evdev" one, commonly found in /usr/share/X11/xkb/rules/evdev. This file is built up using some scripts as can be seen here: https://cgit.freedesktop.org/xkeyboard-config/tree/rules

The problem is that the format is really inflexible about the number of layouts (groups), and so it hardcodes and repeats 1,2,3,4. Hence, if we just lift the limit, some unexpected things may start to happen starting from the 5th group.

Some ways to proceed, from most preferable to least preferable:

• Fix up the rules format to not need to hardcode to group numbers. The linked bug (https://bugs.freedesktop.org/show_bug.cgi?id=14372) suggests an :all syntax, which will work transparently for all groups. This will require changes in both of xkbcommon and xkbcomp's implementations. Also, if the change is not backward compatible, there might be some transitionary issues (a possible workaround is to use a new file instead of "evdev").
• Keep hardcoding the numbers, but hardcode higher, e.g. to 8. This might also need fixing to work; I didn't try.
• Just make the change to xkbcommon, disregarding the above issues. For some scenarios, it will work just fine, I expect (didn't try either). Testing here will be appreciated.

That's it - needs a small effort to push through.

#362 is a first step to support it in rules.

#362 has been merged.