hliles,

Welcome to the Forum!  Your idea is a new feature request that I have written up and submitted on your behalf.  Instruments like banjo and guitar present a programming challenge for assessment because of all the sympathetic vibrations of the neighboring strings. 

For the time being, you can play along with any of the exercises in SmartMusic, import MP3's and change their key or tempo, or record yourself playing.  If you think those features will be useful to you, I hope you'll give a full SmartMusic subscription a try!