I have thought of a simple solution that others may find is good enough.

Use a 0% low threshold.  This is more palatable with ADSM than it was with
WDSF because now ADSM caches the info.  All your data is now safely on tape.

To be sure I'm planning a timer-pop command late at night to change the
hi to 1 and low with low at 0 it should flush everything onto tape.

I'm planning on using this with a prop routine that watches ADSM and
copies the tape to a duplicate when it is 'closed full' and maybe once
a night after I flush the disk stages to tape.

In the end I think there's an ADSM command like WDSF that tells it that
there is some lost data and then , I assume, it causes someone's next
incremental backup to really be a 'new' full backup.  Right?

Of course this all assumes that sometime in the day/or/night theres a
'quiet' period that you can count on.

cheers ..joe.f.