Now what's really needed here is a ruggedised, waterproof disc-based MP3 player that mounts in a bottle-cage, like the batteries for posh headlights do.

Been there, done that. My current lighting system uses a six-volt 4.5 Amp-Hr lead-acid gel-cel battery that I put into a standard water bottle (cut the bottle in half, inserted the battery, glued the bottle back together) and it resides in my bottle cage. Don't know if I can find a 12V battery of the same physical size, though... There are lots of options there.

tanstaafl.

"There Ain't No Such Thing As A Free Lunch"