To prepare songs for this in Band in a Box Windows or Mac, use the tool in Window -> 'BB2Go - Prepare SGU song files for iOS/Android'. It should create a single matching suffixed _render.m4a file with your song.

When imported to the app, the Open - Local Songs, or the Song Picker (tap the song's title in the top bar), will display a label 'M4A+ Ready' on the song's list element. Without needing another generation, the song should play.

As I type this out I think if the song is using a RealStyle, the app may try to download the style file and then prompt for the serial key. This unintended behavior would be a bug, which I can fix. If your device isn't online it might just work.

The iOS/Android apps also perform a special operation when saving on the device: If an appropriately named m4a render file is found, matching the song file name, it will be moved 'into' the SGU file, and the external m4a deleted.