I’v Used it with our own points several times and never had it work. My point was you can use it. It is no more or less likely with DVC than any other reservation
DVC resorts usually do not accept faxes. You put your requests through MS for stays through DVC and through Disney Reservation Center when you book for cash through them.
We either got really lucky or it worked for us with BLT. I entered the same "request" info from the touringplans site into my online check in for a room request. We weren't in the exact room, but we were just two floors directly underneath it.