Yeah, a real accompanist would be good! 😉 One thing you might try is using Audacity, add a new track with a click on it. It would be hard to get it perfectly in sync with the music throughout the piece, but it might be doable. Then when you make an MP3 out of it, have the click track on the other stereo side. That would allow you to turn it up or down as needed anywhere in the accompaniment.
Another way to do this whole thing is to key in the music with MuseScore. Of course, then you have full capability to add anything you want. It is very tedious, of course, and doesn’t sound as good as real musicians. Plus, unless you have a sheet music version that includes the continuo part, you won’t have it. You can make your own by carefully stealing it off recordings (Transcribe! is perfect for this), but this is VERY tedious and difficult. Anyway, Musescore is the most flexible way for making playalong practice mp3s, IMHO.