Hmm... Interesting project. Problem would be, as far as I can see, that the iMac has indeed a very huge screen that is in no way optimised for power savings, as is the rest of the computer. The hard disk, too, is a standard desktop 3.5" SATA drive, very hot and hungry.
It *might* however be possible with a reasonably big battery and if you would be satisfied with short battery life (mayba an hour, tops?). The problems with using 2 MBP batteries would be twofold:

1. The batteries (being Lithium-Ion in design) require internal electronics that interface with the MBP to keep them from overcharging, undercharging and overheating. Simply rigging the batteries together would not only shorten the lifespan of the batteries, it could be downright dangerous.

2. Because the power draw on an iMac is pretty high (up to 220W!), much higher than any MBP, the batteries would drain too deep too quickly, bringing swift death to their charge-holding capacity. That's where those deep-cycle accus would come in, as they are built for this sort of use. They are, however, not only bulky and immobile, but also pretty expensive, especially if you would want 140V (unless you would devise a way to connect directly to the motherboard...

It WOULD however be one of the sweeter mods I have ever seen, so if you could at least succeed at doing the touchsreen, the machine gods will be very pleased. Keep us posted!
_________________________
PowerMac G5 Dual 2.0Ghz, 1536Mb RAM, 160Gb HD, 23" Studio Display
iMac 333Mhz, 160Mb RAM, 6Gb HD
PowerMac 8500 120Mhz, 80Mb RAM, 2Gb HD, 17" AppleVision Display
Mac IIsi 20Mhz, 16Mb RAM, 200Mb HD, 15" Sony Trinitron Display
3G iPod 40Gb
512Mb iPod Shuffle