welcome first of all!

The problem I see is the amount of battery this thing will drain. Its gonna be a lot more than a laptop for sure. If you are gonna do this, I am going to recommend the deep cycle marine battery or two. But, that really puts a damper on the portability factor... The imac just isn't a great candidate.
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"Fix it 'til it Breaks."

Jacob - EiC & Director of Technology
Mac Pro Quad 2.66 - 4GB RAM
160 GB SATA RAID 1 - 650 GB Storage
Quad 19" Widescreen LCDs
Accessorized to the Hilt