Kyle, it sounds like your are trying to replicate the "Vault" functionality of NetBackup, which can be setup to detect tapes which have been used and manually eject them on a daily, weekly, or whatever basis.
This used to be an expensive kludge in older versions of NetBackup, but works pretty well in 6.5. It does require a separate license though.
It is possible to script this activity yourself, but it is not the Volume Pool which you are changing (which refers to a logical grouping of tapes, often by purpose or retention) but the Volume Group (which refers to the physical location of tapes. 000_00000_TLD is the rather imaginatively named Volume Group name for "in our first DLT library", and "VAULT-PRIMARIES-HMED1" is our first off site location).
The commands you need are:
vmquery -rn 0 -b # use to check what tapes in your robot (-rn 0) were assigned, i.e. used.
Then for each of those, use:
vmchange -res -multi_eject -ml T00036 -rn 0 -rt TLD -rh hpstmed001 # Eject tape T00036 from DLT robot 0 on host hpstmed001
Read the command reference, because I guarantee that those command syntax's are not correct!