Success!
I created a wrapper for the BTRFS_IOC_CLONE_RANGE ioctl and manually rebuilt a VHD from RAW images using it. It took about 5 seconds to clone my 500GB image, and the filesystem only shows a space usage difference of 0.03GiB. I shared the result over Samba and it is mountable with diskpart and can be copy-converted to a new dynamic VHD/VHDX using the Hyper-V virtual disk tool. These are the two primary functions I need.
Next step is to automate and, with your help, integrate the feature into urbackup server. I am now porting the raw2vhd tool to Linux.