You can try to disable direct IO, build the package and install it in dom0 for a test.
I have btrfs as well and Windows worked after installing Xen PV driver, but some of the Windows functionality was broken after this, I guess I was just lucky that things that were broken for me were not critical. Seems like the btrfs issue could break some random things in Windows which could cause it to not boot at all as in your case.
Thanks, but it looks I am not in a position to try to do it now. How can I know when this is implemented by the devs?