[schnautza]

Perhaps the more "foolproof" rework would be to remove reference to quarter notes, eighth notes, etc. For the time signature options, simply have a blank for number of beats. Then have a blank for subdivision of the beat, giving options of 1, 2, 3, and 4, (and maybe 6 as well) which may represent eighth notes, triplets, or whatever works for your particular time signature. This makes the "denominator" of the time signature irrelevant and gives more freedom to the user. 

So, for example, in 9/8, set it on 3 beats, and subdivision of 3. Then you set your tempo at what is marked in the music as the dotted quarter with no conversion required.

Of course, for complex meters like 7/8 this means you'd have to input the 8th note tempo regardless since it doesn't divide nicely, but that's no different than the current setup.

Also, it would be really cool to have options for applying specific sounds to specific beats. Say I have a 7/8 meter, I could tell it heavy accent on 1 and light accent on 4 and 6 to easily hear a 3+2+2 feel, leaving beats 2, 3, 5, and 7 as standard click sounds.