I have previously secured Windows NT4 clients running NetBackup v5.x to NetBackup media and master servers running v6.0. I have don erestores too, but not full system recoveries using IDR.
Also, this v6.0 Upgrade Quick Reference Guide:
...clearly shows any NetBackup v5.x client (I think minimum is actually v5.0 MP4) can be secured to a v6.0 server. In my opinion you are good to go. You just can't install/run a NetBackup v6.0 client on Windows NT4.