Many thanks to James & Richard for your input.  You got me thinking of
the problem from another angle and here's what I came up with....

I created a new policy domain for each Domino server, and defined a
new nodename for each as well.  These new policy domains are set to
write straight to the DLT drive.  So now, I'll have to run two full
backups (one for the 'every-day' data, and the other for 'off-site'
data) each weekend, but I'll then be able to restore from these DLT
tapes without mucking with the TSM database at all.

My regular backups that I've been doing all along don't change at all
(still about 5 versions of retention), and handle most restore
requests.  The 'off-site' DLT's are needed only in the event of a
disaster, or when data that no longer exists in the 'every-day' policy
domain is needed.

To my surprise, the backups straight to DLT were insignificantly
slower than our backups to disk -- even during the middle of the day.

This all seems too good to be true.  Are there any pitfalls that I
might have missed?  :)
Eric L.