My understanding is that when you use source/target server, you think of the
"target" TSM server as just an electronic vault.  It is NOT the same as
having a backup TSM server.

As you said, the objects in the target server's data base are archive
objects.  The metadata needed to restore the data is still in the DB of the
SOURCE server.

So you won't be able to do restores, until you build a repair/recovery
source server and reload its DB.  Now someone else might reasonably expect
to reload the DB into a TSM server at the offsite location; but since you
are planning for the target/source TSM's to be on different platforms, that
won't be an option.

You will need to rebuild the TSM server on your OS/390 recovery machine,
wherever that is.