How does the old allegro-dev saying go? The person who writes the code dictates the API? I think it looks fine and I would use it. I think it's a good base to work with anyway. You'll not usually hammer out an entire API and then write the code without any changes. So if you want, you could start writing this in preparation for 5.1. If anybody doesn't like it, well, they've had a chance to say so. And if something turns out not to work well, code is always modifiable. You'd still keep the underlying code in tact so no loss.

I just noticed that rendering fonts with that font created by my function is ridiculously slow. Turned out to be a lot faster when I changed it to make it create a video bitmap instead of a memory bitmap but with a video bitmap the process of creating the font takes several seconds, so maybe there should be a parameter to the font creation function to choose the bitmap type to use.

I think I once replaced the ALLEGRO_FONT vtable so that you could actually render the standard Allegro 4 font. The difficulty then becomes rendering the characters efficiently (didn't solve that one, I was more concerned with getting it to work, so I just used al_draw_pixel). Maybe locking and unlocking the bitmap will help there.