it's easier in my experience to build and check these tables if
you can do it dynamically.  some mappings are more
controversial than others (eg, caps lock and ctrl).  i don't see that
dynamic loading must lead to something complex, if that's
what xmodmap is.

actually, at the moment i'd be more interested in a description
of what the tables need to express to cover all possibilities.
i know there are sequences of escapes in the scan codes (because they
ran out of bits), and keys that modify subsequent characters by
adding accents (on a german keyboard we've driven), and probably
much more.  is there a concise specification?