For a NiMH pack I was not thinking AA size, Eneloop or otherwise. I was thinking a 10 cell brick pack made from industrial type sub-C or larger size cells all shrink wrapped and ready to go. Or a 12V cordless drill battery made from sub-C or larger cells.
That said I think this would work well for him, basically a high capacity sub-C industrial NiMH pack inside of a water-bottle all ready to go. He might even get luck and the plug is the same as his gloves and will plug right in:
http://www.batteryspace.com/waterbot...emaleplug.aspx
http://www.batteryspace.com/waterbot...bikelight.aspx