has anyone ever attempted...

I have a Nomad back in the states with a mostly finished internal lithium ion battery mod. There is a TON of space inside that thing. I used a portable dvd battery pack, so the cells are a few thousand mAh. I'll post pics of the thing this summer.

For anything that originally took internal AAs, NIHM rechargeables are the way to go. I get a ton of playtime on my PC Engine GT using 2000mAh Sanyo Eneloops.