I remember that. A lot of the after market Jeep (and others) bumpers are already welded up air tight when manufactured. A few either use them as air tanks or a water tank. Some have added RV water tanks and pumps underneath the body too. I would be worried about rust thru at some point though. Maybe use POR-15 first? Pretty nice to have when out in the middle of no where with no good water source.
Funny, as I was riding both my bigger bikes today, I was wondering if it would ever be possible to squeeze a Voyager XII 1200cc motor into the Connie frame...