Further to the OP, has anyone tried replacing the battery with an Ipod Nano one? They are (I think) 550maH, 3.7v L-Ion vs the mpx nano clone's 250 or so. They sell on ebay for around US$15 or AU$20 and from comparing pictures of the player internals I'm pretty sure they'd fit.
I'll be buying one shortly anyway - will let you know how it goes.