This logic is premised on MAN containing things like cat.1 foo.8 etc, is that going to also be true for MANGROUPS in general?

yes, it should be. essentially where before you did this:

MAN= foo.1 bar.1

now you could do this:

MANGROUPS= FOO BAR
FOO=foo.1
BAR=bar.1

so ${${__group}} -> ${FOO} -> foo.1 should be semantically equivalent to ${MAN} -> foo.1.