I actually have a didgeridoo, a commercial (schatten) stick-on piezo pickup, and a fishman piezo preamp. Long story short, I tried it and can confirm what others have said, that you get virtually no signal from the piezo because the body of the didg is so thick.

Too bad, because you can make a decent,cheap piezo pickup from a $2 radio shack buzzer:

http://www.radioshack.com/product/index.jsp?productId=2062402

You could try putting an electret mic into the end, one of these and a 9V battery will do the job:

http://www.radioshack.com/product/index.jsp?productId=2062216