Yes, I have been thinking about doing the Piezo thing,but have not yet come across piezos that sounded good passive(I don't want batteries in my bass).
Well, I may have , but I have not yet had a chance to try them.
Which system is being used on the MG bass you mention?
as far as in know, there are impedance issues which mean that you can't have a completely passive piezo p/up. that being said, the way i mix the mags/piezos in my basses is using an outboard device so i can also go completely passive (no device/preamp) when i want (which is most of the ime these days). piezo goes to one side of the stereo output jack, mags to the other side. use stereo cable to raven labs pmb-1 (d-tar also makes something like this), which is two channels and i can mix to taste. the only thing i don't like about this arrangement is that the pmb-1 (and i think the d-tar thing) puts out LINE level, rather than instrument. kind of a pain. i talked to mike and david yates (last name?) about making something that didn't do this.

