Zuh? Why in the world would you go to the effort of copying all files from a tape drive except what you want deleted to somewhere else, and then destroy the physical tape the 'to be deleted' file is on?
I mean, I'm not a programmer by profession (although I have dabbled in it for years), but I can't imagine it would be that hard to make a program that writes over the data of X file with zeroes. *BAM*, problem solved, no wasted time or items.
Isn't there even a contest of some sort in which a hard drive is overwritten using a *nix command for overwriting it with zeroes that noone has ever recovered data from? Ever?
Hell, at the bare minimum, go with your original theory of copying the 'good' files elsewhere, and then use that 'overwrite' command on the tape! There, reuseable!