Not an answer to your question, but an alternative suggestion for encrypting a tape backup.
You could use the FUSE filesystem
encfs in reverse to mount a local path to a special folder, where the contents of that folder are on-the-fly encrypted versions of the source path. If you
tar that folder, you'll be writing an archive of encrypted files.
When the time comes to restore the data, extract it to a folder, run
encfs normally, and then you will be able to see all your original files.
The main drawback with this method is that the filenames are also encrypted, so you are unlikely to be able to restore just a few files from tape - you'll have to read the full tape back onto local disk, copy the decrypted files you need, then delete it all again.
An advantage is that
tar only sees normal files, so its multi-volume handling will work well and split files across tapes. You can even restore one tape (or do a partial restore) as
encfs files are self-contained (one encrypted file matches one real file), so no matter how many or how few encrypted files you get back off the tape, those files will be able to be decrypted. You just won't know what the filenames are until after you've copied them off the tape.