Yes, MIDI clock is fixed at 24 pulses per quarter note (crotchet)
tgoose is right that BPM is not really a formal concept. In those modern scores that give a numerical tempo indication, this is now done by something like “[crotchet] = 110” or “[quaver] = 90” where the bracketed entity is the actual symbol. In a compound time like 12/8 you would see the “beats per minute” referenced to a dotted quaver.
The widespread use of BPM came in with early drum machines and their use in disco and later dance music. For the first time it was meaningful to put a precise indicator on a label of the music’s tempo - useful for DJ’s trying to assemble a set list. Since 99.999999% of dance music is in 4/4, there was no need to reference the BPM figure to a note length - crotchet was implied.
Moreover, drum machines always referenced their “BPM” setting to a crotchet. Anyone used to programming a Roland would expect the transition from 4/4@120 to 4/8@120 to be a switch to double time feel, and probably describe it like that.
It probably comes down to the backround from which one arrives at a DAW. Ex-beatbox programmers (like me!) instinctively “see” a numerical tempo indication as defining the crotchet. You might say our brains work in MIDI Clock. :-)) Someone with a formal music background might well see it differently, more in line with the current Ardour implementation.
However, this could be resolved by just making the beat indication clearer. The issue, to me, is more one of workflow. If I want to insert a bar of 7/8 into a stream of 4/4, it is almost certainly because the effect I want is the “surprise” of a missed half-beat. I’m really inserting a bar of 3.5/4! At present, to get the correct effect, dmoore and I have to insert tempo changes either side - otherwise, the bar will sound to the listener as one of 7/4 with a half-time feel.
So, if it proves to be too difficult to offer two different modes of operation within Ardour as regards the meaning of “BPM”, then another tack to take is to simplify the above task, by providing the means to automatically add the necessary tempo changes. All this needs is for the project to have an idea of “prevailing denominator” (= that at the start) and, upon inserting one more measures with a different denominator, to offer the option of a “fudge factor” associated with that measure that frobs the BPM so as to maintain a constant crotchet (MIDI clock) size.