Originally Posted by Zorak

It might be agent-dependent, but I imagine it would be possible.

While acknowledging this isn't the same thing, I have added GUCs (waitlisted) via Twitter in the last month. They might ask you to provide the same authentication info they ask for on the phone. (They didn't in my case, possibly because I had just selected the GUCs and they weren't visible to the agent so I had to give the full 006 number and redemption code anyway)