Maybe not a pony, but nearly so, my welsh section D is ridden in a treeless, a Freeform, and he's absolutely fine in it. It doesn't have spine clearance, just wither clearance, but given that you're not putting a piece of wood (tree) on his back, and the numnah doesn't have a seam on the spine, is it that vital? He's never shown any signs of a problem since April when I bought it. Bebe is right, he sometimes sweats up like any horse, but never down his spine.
My Freeform is black leather,and looks fine, I think, although not as conventional as the Heather Moffat one. It does look more conventional than the Fitform did or the Torsion does. I understand it's Ok to use for dressage or the show ring, but I must admit I haven't put it to the test.
Drummer's mum, I also have the fat bum effect, and found the 18in seat is absolutely fine for me! The advantage of having a saddle with an interchangeable seat! I sound like a walking advert ..I don't mean to, just am very happy with my saddle.