Thanks, Charlie. After a lot of experimenting and thinking, I think I'm understanding the situation. On Windows, BIAB would have the Roland or Coyote DXI synthesizer and I think that perhaps it can render faster than realtime (which is what BIAB does). But DXI refers to DirectX and that's a Windows thing, not a Mac thing, so there's no direct equivalent.

On Mac, it would seem, the only MIDI sounds that can render faster than real time would be those directly available within BIAB and they are not as classy as the sounds available through using a software synth.

I could just record the audio in real-time from BIAB, as you suggested. That does require more time but the main problem is that for my particular application, it's important that the starting time be accurate for the track. Perhaps there would be a way to make the recording start at precisely the time BIAB starts playing, but I don't know how to accomplish that without doing a manual trim afterwards.

So, for me, I think babarton's suggestion is best and I've already tried it using Logic Pro X as the DAW. My strategy will be to make the real tracks accompaniment and the MIDI "melody" track sound as good as I can in BIAB. Then I'll drag and drop all the tracks to Logic Pro X and be able to substitute a Garage Band or similar instrument within Logic Pro X, adjusting the mix and making small changes as necessary. I'll still be able to produce files at different tempos, provided I mark the audio tracks as "Enable Flex" after bringing them into Logic Pro X.