Since the code is included, it might be possible to compile and run it through Cygwin?
I'm not sure if it's possible to build binaries using cygwin which will then run without having cygwin installed. If not, this would turn into "port tarsnap to Windows", which is also on my to-do list, but much lower down.
One client I'm dealing with now has health care data needing backup. I wouldn't trust other online backup services with this, but would trust tarsnap. For now, we're using USB hard drives.