I hope Salty responds to this thread, he knows more than the rest of us. For my 2 Euro, I think that you can, if only in the back. Following my involuntary mobilization a few years ago, my ID expired while I was out and I was out of luck and my airline didn't see any reason to give me a new one until I was headed back to work. I read somewhere that you should be allowed to jumpseat. You can check out the USERRA, it's a bit long and dry, but full of information. Another great resource would be to check out the
www.roa.org website and look under the law reviews. Also, I'd press your mil rep at FedEx, he should have the information you need. Good luck.
MBB