Anchor drywall edge to ceiling next to a wall running along joist?
You can purchase metal "corner backing"--essentially a sheet metal angle that you attach to your top plate. It's very similar to corner bead, which could probably serve as a substitute, but it doesn't have holes and it's slightly larger. The drywall then screws into that. 

Obviously the screws will penetrate the air return, but I don't consider that a problem. 

There are also clips like this that you'd slide the ceiling panels into. They'd be oriented opposite of what's shown here.