My dressage and jumping saddles, which once fit my horse, are now rotating to the right at the cantle when I ride. He has had some issues which have created some asymmetry. I am trying to get this taken care of by a saddle fitter, but it may be a long process, and want to see if there is something else I can do. I believe my saddle, a Barnsby Bonfire, has what I have seen called y-shaped billets - the billets attach to a v-shaped strap, and no strap attaches directly to the point. Any thoughts anyone may have about relocating the billets and/or whether an anatomic girth may help resolve the rotation issue would be greatly appreciated.
I would be more inclined to spend my money on a good shim-able corrective saddle pad before trying to strap down saddles that don’t fit correctly. And then I’d get the saddle fitter out. 
yes, and a no slip pad as well!
Your situation is a little different than mine because your saddle is rotating as a result of the horse’s movement. Mine is a treeless, and my horse has no withers, so mine simply rolled.
I cured it with a Midnight Dixie Pad – not too expensive, and thin enough to put under whatever pad you like to use – and a Montana Cincha string girth.
The combination does work for me. No more rolling. 
OP - I use an anatomical girth because my mare has a forward girth groove, and it helps keep the saddle back from her shoulders. I don’t see how this could really help w/ your problem, and they are expensive, so I’d borrow one to try if you can before buying. I think more likely shims and something nonslip until the saddle fitter gets things worked out…
There is an interesting article in this month’s The Horse describing how hind end lameness often manifests as saddle slip. Worth looking into, especially since you note that it’s not the whole saddle (which could be caused by you), and it’s two saddles with the same problem.
Here is a link to part of the study they spoke about last year. http://www.thehorse.com/articles/29841/saddle-slip-as-an-indicator-of-hind-limb-lameness
