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.
When I bought my mini truck in 89 my dad gave me a length of some 3" drill tubing from the oil rigs, it had a 5/16" thick wall. We build the rest of the bumper out of 3/8" plate... Needless to say that when I once got rear-ended on an icy road the Chevy front bumper simply folded under and I got shot forward like a pinball with only a scratch in my paint....