Interpreting time signature as quarter notes per minute

Hey hey,
is there a way to interpret time signatures as quarter notes per minute, 
rather than as beats per minute?

Example: with a time sig of {6 8} at 120 BPM one 8th note lasts 0.5 seconds, 
so that there are 120 8th notes per minute. The metronome I sync to externally 
from my DAW would calculate it so one 8th note lasts 0.25 of a second, so that 
there are 120 quarter notes in one minute.

One could double the tempo in Midish and use a 6/4 time signature externally, 
but that leaves a small margin for higher tempos.

This example could possibly be adapted to use a time signature of 3/4, but I'd 
rather like to use 7/8 or 5/8.

I'd apreciate any suggestions.

Best wishes and TIA,

Jeanette

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