From what I can see on the interwebs, the FiiO definitely fill the high power output requirement. I'm not sure how long those 3,700mah will last under those conditions tho. The let down was the lack of internal memory from what I saw. And the price, $349 on amazon... probably because is a high fidelity player.
It seems, from the reviews, thatit does play mp3. It's just not marketed towards that. They seem to be going for the people who want to play loose less formats.
For that price, I would get a Zune with a HUGE internal memory.. or a Sanza and a super-huge portable usb charger.
"Hoy es un dia normal, pero yo voy a hacerlo intenso" ~ Juanes