I am considering this setup as well. I do not yet have the backrest, but my Corbin is setup for one. Was it easy to slide it back and can you only do that with Phil's rack? I assume from what I'm reading here this will only work with mounts like the one on the Kawasaki box and not the Givi.
Slidding the trunk back with my setup is very quick PH, I also have Phil's T-nuts, just losen from the top and slide back and re-tighten, done!
As for other trunks I can't say because I haven't tried anything other than my Kawi, but my guess would be that you
would need a mounting plate of some sort like the Kawi uses, a direct to rack mount I don't think has moveable mount points, but maybe I'm wrong?
I think you'd really like this setup bro, I know I do and so does mama.