I was successful in using my DVA to pay for an OBB on the Magic a couple of weeks ago. The woman at the desk (I think it's vacation planning desk or something now instead of "Future Cruises") was super on top of it. She said it was totally no problem as long as I had the full number. She took the number, was able to check my balance, gave me the cost of the deposit and even told me what my remaining DVA balance was. We were in a US port the next day (Key West) so I was able to log in with my phone and check that it was debited. My wonderful TA (Hi
@IBelieveInTheMagic!!) then took it from there. It was flawless.
On the other hand, I tried to use a gift card for my OBB on the Wonder in Alaska and it was a mess. She didn't use the gift card somehow and it ended up being charged to my credit card on file. I wasn't very happy, but thankfully had kept the gift card safe in case of any issues. This lady seems not as with it and was flustered. She told me there was no way to have a print out of my reservation (which isn't true because I asked on the Magic a couple of weeks ago and she printed me a confirmation no problem).
So it's super hit or miss, apparently, depending on how knowledgeable the CM is.