Sorry I didn't get back to you sooner. I posted this thread and then it sank without a trace and I figured noone was interested. And then I haven't been on Mudcat so much recently.
So, if you want to try out JamPlay for free for 7 days, go to:
and enter the following code: 5E98941F20
And Kat, unfortunately for you, the site is completely devoted to the guitar -- no other instruments. I guess they figure they should stick to one focus. I assume there are other sites like it for other instruments, though. I bet if you searched around you could find some.