they are defiantly allowed in affliated competitions, anne bondi (who has ansurs) competes in eventing
http://www.ansursaddle.com/WhyGoTreeless.htm
i dont understand about the back bone being to promenant thing, my tb has huge withers and is a little underweight and she has always had a treeless on, with th correct padding its fine (i know most people think you just plonk them on as they are)
this is maddie, my ex racer (the one in the pic above
just to ad that i got mine kk secondhand and got it for £850, im not sure if i was lucky tho, seeing the price that soem of them have been going for