I like Jay's idea, make a cat cage around the birdhouse, far enough away that the cat won't be able to reach the house with foreclaws. You might have to come by with a water bottle a few times per day if the cat parks himself there.
Depending on how motivated the cat is, using some chicken wire as "tanglefoot" a few inches above the ground might discourage him from even climbing the plant.
Or make a tent of chicken wire around the plant. I think it would be too flimsy to let the cat climb it, and I don't think cats like getting their paws entangled. If it came tumbling down on top of the cat a few times, he might give up.
