So, you could recall these part-filled, potentially appendable tapes if you needed to?
I have similar scenarios ongoing, with the exception of having to offsite tape. I am lucky enough that my environment has a library/robot on sites A & B, and we take 4 copies of data - Primary Disk 1, aux's to Disk 2 @ Site B, Tape 1 @ Site A and Tape 2 @ Site B. So plenty of tape devices available. Bear with me, it is relevant.
A couple of our SPs were set up and for some reason (I forget why) did not Append to Tape. So every new schedule would start new tapes...so 1.6TB media might have 40-50GB written and not get appended!
So I changed the config to correct this, then setup a new copy which essentially ploughed through all the part-used tapes, and copied all their data to a single (at first) new media, then used that copy for future aux from disk when it had fully caught up. OK, this required 2 tape drives but I combined 20-30 'part-worn' LTO4 down to a single one.
I used the same technique to combine a *LOT* of LTO2 tapes (data with 7yr retention), on to LTO4 tapes.
The point is, if you can convince your managers to work with you on the offsiting and recalling of the tapes, you can resolve your situation. You may have to do it a few times, but you don't need special features, just creative thinking. And no, I don't work for CV... ;o)