Hey Eddie,
Do you think something like this might work for you ?
https://www.macupdate.com/app/mac/11333/linein

You could try route the line in directly to your output device with this app, and then launch BIAB and see if you can play along with BIAB's audio output without record enabled.
I'm not sure this would work, as I haven't attempted it myself, but maybe it's worth giving it a shot (it's free).

Yeah, yeah, yeah...
I know it says 10.7 only, but if you scroll about 3/4 of the way down on this other page you can find a version for your system (I posted the other link because it gave a better description).
https://rogueamoeba.com/legacy/

Like I said... I'm not sure if this will do exactly what you are trying to do, but doesn't cost anything to try.
Hopefully this helps !

- Jay

MacPro 5,1/12 core@3.46GHz -- OS X 10.14.6 -- ATI 7970 -- 32 Gig RAM -- Crucial 500GB OS SSD -- Samsung EVO 1TB Audio/Sample SSD -- BIAB 2023 -- Logic Pro X -- Cubase Elements 11 -- Too many plugins