I think I am looking to do the same thing or similar.
My setup looks like this
1 RedHat Enterprise Server 5.3 - running netbackup 6.5
I have 1 ML6010 tape library and 1 ML6020 tape library connected.
WRT policy I have two types of backups currently configured. One set of policies sends .img files (and associated header files) to the hard drive at /var/Backups_DSU. A second set of policies sends jobs straight to tape.
I am able to use use mt and netbackup tar to find the contents of my backups once they are on tape but what I really want to do is pull off the .img file in its .tar state.
I want to grab the .img file because I want to compute a digest (with a hash tool that will product a md5, sha1, etc). I want to be able to go to my tape archive library 5 years from now, find my tape, insert it into a system, confirm that the digests match, and then work on my data. Primarily for forensics purposes but also as a classic integrity check before I use it to restore. I have been looking for a good document on how veritas ensures integrity but haven't been able to find anything meaningful.