I actually was able to download an SGU + M4A file pair to "mobile BIAB" on my iPhone and play the song!

But you need to point to a BIAB server running on a Windows desktop system – not a Mac – to do any editing or generating on the iPhone:

https://www.pgmusic.com/support.iphone.htm
(note the separate "Mac users click here" page)

https://apps.apple.com/us/app/band-in-a-box/id397173717