From the research I've done, it sounds like there is a debate if it's safe or not.
From what I read the angle of the bead on the HMMWV rims is 15 degrees, while a normal one-piece rim is 5 degrees. Either way the air in the tire pressing against the bead should hold it solid.
When originally mounting the tire, it will be harder to do the initial air-up as the bead won't be solid. And I will NOT use the ether or other dangerous inflation techniques. The inner beadlock (PVC or runflat) will press the bead to the rim and help seal this so the initial air up is much easier.. but even then...
The real different comes when you lose the air, I don't plan to 'air down for off-roading', but instead worried about a puncture while driving. Due to the additional 10 degrees the tire is more likely to lose the bead. Best case difficult to air back up, worst case it could work it's way off the wheel and cause damage to comeone else. (I don't know that I necessarily believe this part of it, BUT it's been reported to be a concern.)
From this, I decided that less then 20 lbs for an $80 PVC insert, per tire.. was a small price to pay for easier mounting and possibly less likely to have a bad accident in case of a puncture. (I don't want to deal with the runflats, tire grease, balancing, etc etc etc.. so this was a good compromise.)
There was also a post I saw that someone indicates that the rims are only DOT approved when an internal bead lock is present. I could not find anything that ACTUALLY indicates that is true though. (Nothing on the DOT website indicates it is, that I found.)
So.. in short. Running the tire without an insert or any type? Absolutely works.. it works for a lot of people. Is it safe to do? As long as their is air in the tire it is probably just fine. The question comes in, when air is lost is there a danger -- not sure. Is it difficult to put air back in the tire if the bead breaks free? Absolutely.