This sounds like a very cute trick to train! I found two very cute videos on youtube of birds doing it. I don't think it would be that hard of a trick if you've already trained behaviors like playing dead and rolling over. The only pre requisite would be having a bird that's comfortable going onto it's back on cue.
http://www.youtube.com/watch?v=DXVawT9i3wA&feature=PlayList&p=FBD7317CA3EFF34A&index=12http://www.youtube.com/watch?v=F8DQGZZzd2Y&feature=relatedThis trick wouldn't have to be trained to a clipped bird that you were forcing to do it. You would definitely need trust and a good training relationship with your bird though, clipped or not.
If I was going to try training it I would use luring. Use a favorite treat and start by luring the bird's head down towards it's feet. Eventually work so the bird is putting it's head in between it's legs further and further. You might have to help with the initial roll over just like when you're teaching the play dead trick or the roll over. Gradually decrease the luring and physical manipulation, add your cue, and you're done! Sounds easy in black and white
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This might be the next trick I work on after we've mastered stick 'em up.