ive seen it done on youtube and it seems to work well. your talking about fastening a magnet to the branch on the inside by hot glue or somthing and sticking another magnet to that one on the outside right? i would think it would be better for cleaning and stuff as long as they hold his or her weight.
I think it's a great idea. If you're looking for really strong, small magnets, go to the hobby store and ask for "canopy magnets". They are used in remote control airplanes to hold the canopy in place. They are super strong and no bigger than a pencil eraser.