Please allow some reasonable level of customization of metronome.
I can think of one possible way of doing this.
How about letting metronome follow the hint from the "Tempo" designation. Now, it seems to me the metronome simply use the denominator of the time signature.
For example, if user say "quarter and a half note = 80", let metronome beat at every quarter and a half note. This would be quite useful for the 6/8 measures, counting two beats per measure. Actually, this following the tempo designation scheme would satisfy most needs. For the weird case of odd numbered measures, e.g. 7/8, user can carefully designate tempo to count 3, 2, 2, or whatever.
Or, you can certainly add a new (and more convenient to implement) way to customize metronome.