How should I treat a rusty corner bead in the bathroom before covering it again?
You want to take a moderate to fine grit sandpaper and try to sand as much of the visible rust off as possible around the outside corner.  Don't be concerned about plaster or drywall damage just yet.

Next get a good rust sealing primer like this product:



Spray liberally along the outside corner until the metal is fully covered.

Next, spackle then sand the damaged areas and hit the area once more with a regular latex based primer.  Apply two coats of paint to match the wall and you should no longer have problems with the outside corner.