Neat idea, eh?
A mini, 2/3A, pack might fit, but maybe not. One of those buterfly "AA" sized packs would fit for sure. Obviously this is designed for looks...and not necessarily for all-day, no-battery-swap play. Battery bags are fugly.
7.4v Lipo might fit. Butterfly LiPo's would for sure.
The Patriot Long Sniper kit is awesome looking. The long fluted barrel is really nicely finished. The one thing I don't like about that particular kit is that the "cutouts" are pretty much the entire length of the long handguard...so there's some side to side flex of the outer barrel. If they could fit, you could put at least 4 minis (2 on either side) in the size of the cutout.