Although I do have a cheaper alternative than an expression system... There's a few floor effects boxes out there which will warm up the sound of a Piezo pickup and make it sound "close" to natural. Both in a live situation and on a recording. They don't sound as good as the Expression system, and they don't sound as good as truly mic'ing the guitar properly, but they are decent.

I used this Yamaha unit for some of the acoustic recording bits on Blindsight. I only used it because I was stuck doing emergency last-minute recording without a studio. If I'd had my druthers, it would have been done with proper microphones, but that just wasn't available to me at the time. The results of that recording can be heard in this MP3 sample from Blindsight.