I agree that it is highly unlikely, but I'd have said the same about the Mavs trading away Luka.
I highly doubt that the Spurs would be interested in AD. He is well past his prime.
Going after a PG also works as Harper is probably the best available, but I don't see how he fits in with Castle and Fox.
This is probably just wishful thinking on my part, as a Spurs fan, but a lineup of Castle, Fox, Barnes, Flagg, Wemby has a chance to be extremely dangerous, and is probably Barnes' best shot at getting a ring.
You also have Vassell coming off the bench, so the Spurs are already extremely deep at the guard spot, regardless of what happens with Paul.