Mark, You could make an 'Archive' schedule or policy and combine that with a separate 'Archive' Volume Pool. Then, after you run the job, eject the tape and put it some place safe. Or leave it in and put other 'archive' jobs on it as well. Just make sure you configure the schedule/policy to not do any multiplexing/multicasting and you're good to go.
Mark, I get requests for special one-shot backups with long term retention all the time. I have a pool called archive that I use. I create a policy using the name of the remedy ticket that the request came in on as the policy name. I place a new tape in the archive pool because I don't want it pulling a random used tape from scratch because I want to make sure that the data is recoverable for many years. If a tape has already been in use for a couple of years then I don't want to use it. I prefer a new tape.
Just an extra step that makes a regular NetBackup admin look like an expert when you tell the customer about the care you are taking in honoring their request. :)