If you just want to make covers and take the time to do it I'd suggest RealBand. Band in a Box is designed to change the bass line every time you press play. It's what it does.

You can freeze it, move it to the Soloist line, mess with it, and use a midi instrument to play it. You cannot get RealTracks to play a line as printed, it does not work that way. (At present).

There are covers out there of almost all the stuff you'd want. If you find the right one and a midi module you'll fool 90 percent of the people. Of course someone in the audience played a zither and they know for maybe that's not really one, but maybe one made by a blind guy in MN. When the dogs fell through the crust on the snow and stuck you in the cabin hard by the Canadian border, the snow is horizontal and your parrot, though stuffed, agrees with your rendition of Bob Marley plays parrothead music. Nice.

John Conley
Musica est vita