so I'd research it quite thoroughly to make sure it wouldn't damage the Time Machine backups
Like Csound1 did.
A Windows backup would probably like the have the hard drive formatted for Windows, but I'm not certain.
You're right regarding the Format (Windows Backup requires NTFS) so the TM drive would need to be partitioned for both OS's.
They'll still end up in each others way, as the backups grow space will be at a premium, an external drive is so cheap I would get one for Windows (or if not much data in Windows to backup) a thumb drive.
Windows System Image must not be used to restore to Boot Camp (it creates it's own partition, thus borking everything else on the drive)