Well I guess if you  had  caching turned on in the diskpool and then
migrated to tape and then backed up the tape pool  you would have three
copies of your data at the same time.
Caching isn´t always a good thing though so if you could set up a Diskpool
for these logs and just backup the logs you spoke of to this Diskpoolpool
and turned the caching on in this diskpool you would get the same result.

Other than that I think that if a Tape fails during the Migration then the
migration fails and the data won´t be deleted from the cache(Committed to
the DB)
But maybe I´m wrong.