That would be nice to have. Why I want a global time signature is because it seems like the only thing that would allow to maintain your usual workflow no matter what time signature you happen to choose.
Even if there was a .75 multiplier I would be going: Ok, 808 page track 1: set multiplier. 808 track 2: set multiplier etc up to track 10. Repeat this for the other drum page. And the bass page and so on. Very tedious compared to setting a global sig; BAM.
A global time sig would probably also be the only way to do odd sigs like 5/8 and 7/8. By that I mean that I am not sure that the clock resolution will allow for a track multiplier that plays 14 steps in 16 steps time.