Forget about using Audacity for that......It might do in a pinch but it's not designed to be used as a multitrack recording DAW. Use the right tool for the right job.

I'd set up Real Band for that purpose since it is a DAW.

Get the song the way you want it in BB and then save it and open it in Real Band. RB will create audio tracks for the real tracks and use synths where it must.... If you use the real track styles in BB there should not be any midi synths in RB's rendering..... which is what you want.

Now...simply select an unused track, set it up for audio recording as mentioned above..... and hit record and see what happens.

Yes, most of the folks in the showcase, especially anyone singing on the song is doing exactly what you are discussing. Some folks, like me, are using multiple live tracks recorded along with the PG tracks.... a mixture really sounds good. Probably the biggest downside to playing live tracks along with the PG real tracks is trying to sound as professional and creative as the RT's sound. After all, they are some of the best session players around.

I too take all my tracks into Sonar for the final stages.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.