Steve,

I know that many external soundcards come with software FX (reverb is usually a staple). So, if your soundcard has FX it should be fairly simple. You open BIAB and plug your mic into input, most external soundcards have a gain knob, and you adjust your mic level to match how you see it fit with BIAB arrangement.

Possibly you can run your mic through BIAB FX (not sure). As you mentioned, you just want to "play along" and not record...