i used to gymnastics when i was younger and could flip i have lost it over the years but i am pretty positive the harness thing will only get you used to being upside down and wont help learn how to do flips... mainly because you need your arms to get the spin and the harness would get in your way of doing that...
and i know you can flip with out your arms but when learning they are pretty integral, and big motions, got to admit it even when i get back into flips on foot i would be warry with the added weight of the PJ and the added spring they give the jump along with the fact you will have to land correctly down on them or your on your ass...