I was able to create a test percussion loop in RealBand and use it in BIAB. Here is the detailed info.

RealBand:
Note: there is help in the real band manual PDF. Just press ctrl+f to search for Acid Loop.
Create the loop in RealBand on a separate track. Play it against an existing BIAB file which has been imported into RealBand.
Record your click track on a separate track the normal way.
You need to highlight the loop from the end of the count in where the music starts and highlight at least 4 beats (it depends on what you want to do).
In the popup (that appears after you choose the directory and name) the speed must match what you recorded it at.
You set the time signature to the proper time signature you recorded it at.
Leave "One Shot" unchecked. I assume this is because you want to allow tempo stretch in BIAB.

BIAB:
Bring up any song at a reasonably close speed (maybe within 50 BPM).
Choose the track where you want to insert it.
Find the loop in BB\RealTracks\Metronome (or whatever). You can search by name to find it.
I chose to trigger the WAV every 4 bars (you can do whatever you want).
Make sure that "stretch tempo" is checked.
The tempo should be the original tempo (at least that worked for me)..
#beats to stretch can be 55 or whatever. If you apply it to a song much slower you may find it is not that accurate.
Uncheck "transpose" because it will be a drum loop or percussion loop. Your guitar click track is either (a "real track" or a "user track" you created yourself) rather than a loop.
Click regenerate and play so it picks up the new loop and plays it. Clicking play alone will not pick it up.

Last edited by bowlesj; 12/09/18 05:22 PM.
John Bowles
My playing in my 20s:
https://www.reverbnation.com/johnbowles