I have never tried checking a 5 gallon keg. I have checked many odd things both successfully and unsuccessfully.

I'd expect to get:
1) a lot of heat and lot of initial "NO"s
2) an open hand asking for a bunch of special handling charges
3) at the very least hassled quite a bit
4) "I don't care what .com/the call center told you, we won't do it here"

Of course, you may not get any of these and it may go well, but if you expect any one or combination of these, you'll at least just be pleasantly surprised. So I guess that means:

1) Get there *really* early if you're gonna try it.
2) Know how much you're willing to pay (and be prepared to pay)
3) Have a backup plan (e.g. when they deny your keg boarding, it won't get IDB ).

Personally, I've been through too much crap over the years with non-standard baggage (either hassles, payment demanded, being very unhappy with what it looked like on the other end, etc). If it isn't a rectangular black suitcase with nothing dangling off it, I ship it with a carrier whose business is handling packages. It's cheaper the overwhelming majority of the time, more reliable, and door-to-door. Bottle a few bombers and hide them deep in your checked bag if you want some instant fun when you arrive .

UA Cargo, UPS, FedEx, etc all will ship alcohol to places it is legal to transport that amount, pretty economically too (well, except UA cargo, I doubt that is not economical in this case).

As for the pressure arguments, we can all do the math and know everything is a-ok, but UA's liability policies and internal policies won't trust us. Besides, pressurized canisters (of any pressure whatsoever, excepting consumer cosmetic products) all pretty much fall under some kind of DOT regulation or another. The regulations for shipping pressurized containers in compliance with standards are stringent enough/expensive to comply with that the only way they'll ship containers capable of pressurization in general baggage (e.g. no special handling) is if they're bone-dry/empty and the valves are open (e.g. scuba tanks) and even then, it is $175 (for one scuba tank, empty, max wt 50 lbs). The gate agent/baggage agent isn't going to entertain the physics lessons .