Interestingly enough, while talking to reservations this morning, I asked how to ticket with one of the UA DB vouchers. Agent told me it had to be mailed in now, and I said I always prefered to ticket at the airport as I didn't trust USPS. She said you couldn't, then qualified with - agents at the airport are very busy learning the new system. I don't know if she was trying to get at that was the reason why, or they literally can't (or won't) do it.
On the other hand, if it can be done at the airport, maybe the new system will stop making them call Visa to type in some sort of verification code - this has happened the last three times I've tried to ticket at the airport (and I've paid with MP visa card).