Well, if those LUNs are for an AIX host, then I see no harm in trying to find a way to back up the data to an MVS attached tape library.
And IF the LUNs are in AIX volume groups, and IF these VGs hafe filesystems in them, then why not back them up to TSM on MVS. For AIX, TSM is a server, the platform on which it runs is irrelevant.
I'm not sure if LAN-free backup is an option, because then I think TSM client and TSM server need to be on same platform...
HTH,
p5wizard