Good idea, it does sound like she may be collapsing slightly over the hip to one side and weighting one seatbone more than the other.
Re: treeless, I have a Heather Moffett FlexEE which has a flexible leather tree, and is completely adjustable. I love it and, more importantly, so does the pony, who moves beautifully in it. Saving up for a second-hand Vogue now!