Well that's not the sort of thing I had in mind. All this does is provide a DC bias to the sensors to allow negative voltages to be measured. It still has the same problem of having a 1M input impedance and that is what is causing you the problem.
Look at this thread and the links off it:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1239839000
The sort of thing I was thinking about was a voltage follower:-
You can reduce sensitivity to this AC pickup by sampling the piezo element at exactly the mains frequency
That is just twaddle, have you ever tried this? A simple minded look at the theory says it could be static if the interference is mains. But the size will depend on the phase relationship between sample point and mains pickup. The mains pickup phase will change with a persons position so even if you could synchronise it exactly it will not work.