I guess you know about that:
http://mikerubel.org/computers/rsync...ots/index.html
I think you just have to follow this how-to, and make it so that it is the backup server that takes the initiative of the backup, and gets to the files. You would then mount on the backup server all remote places to backup, via NFS or SMB, and do the backup.
As a consequence, the things to backup on windows simply would have to be "shared", and the things to backup on other OS would have to be exported via NFS.
Yves.