Ahhh...I had it until that last one. Crashplan meets all of your requirements, and does it for free for local backups, but it can't back up to a network share with the Windows client. I've seen solutions that use symbolic links, but I think I've tried those before and they always seem to fail at some point. I'll let you know if I think of an alternative.

*edit*

I forgot that I've also tried a trick where a batch file is set to run at startup that connects to shares as the local system user, making the shares visible in Crashplan. This has also failed me, but maybe you can get it to work.

In the end, my entire model shifted so that all my data is on my NAS, and I went through the annoying steps of setting up Crashplan to run directly on my Synology. It's not the best solution, particularly because it disconnects from time to time, but it works well enough for me.