I did look at that when having the 404 problems on GOG with D:OS - unfortunately there's no precompiled Windows build and I didn't fancy getting down and dirty with MiniGW.
If you don't have a Linux machine running, you can always make a VM and compile it there with gcc or clang. It shouldn't be hard.
I think that's not a reasonable thing to expect from them, since such updater client is a positive thing (it improves usability). Its problem is in the lack of trust. So it's more reasonable to ask them to open the client or at least to document the protocol.
