I might be wrong but, I believe that freezing the tape may cause you to have to do a bpimport of the images that are on the tape when the images expire. True that it doesn't delete the media from the media catalog, but it does allow the image to expire and be removed from the image catalog. Two separate dtatbases. It is also true that the tape will remain in an "assigned" state. That is so that it will not be used for another backup even though all the images on the tape have expired.
A better solution is to use the command bpexpdate to recalculate and change the expiration date.
legal values for sched: 0=full,1=differential incr,2=user,3=arch 4=cumulative incr