Hi,

Your idea should perfectly work.

There is an maybe even simpler workaround:
        update stg your_primary_tape_pool access=readonly


But using any of this there is a danger your disk pool becomes full thus 
preventing any backups from succesful run.
I find the risk of occasionaly having data migrated prior
to backup stg pool is smaller - in such case you have only
one copy of backup files, but without backup you have no
copy at all.
You still can periodically try
        backup stg your_primary_tape_pool  your_backup_tape_pool preview=yes
to find out whether there are files in one instance only, and if, what tapes
are they on.
In this case you still can move data from this tape volumes back to disk
pool,
backup and migrate them again. It costs much time but is quite secure.

regards
Juraj