I was the OP, and thought I'd come back with an update in case anyone is interested. My in-laws had no problem buying the tickets for DLR in Virginia where they live. However, as some others have reported, they had to "order" the tickets for pick up about a week and a half later. When the tickets "arrive", they will have to go back to the base to pick them up...and they are going to be paper printouts rather than actual tickets. The print out will be exchanged at DLR for real ParkHoppers (although it's unclear to me at this point whether we can present it at the turnstile, or we actually have to go to a ticket booth for exchange).
It's puzzling to me that they can't figure out a way to do a computer print out when you are standing there rather than it taking over a week to do so. Is that our military bureaucracy in action?