29 Nov 2021, 11:20 by phill@thesusis.net:

Theoretically it shouldn't be too hard.  It's just a matter of deciding
on a location where you can safely record the checkpoint information and
then update the checkpoint between blocks.  That's how LVM handles
moves safely.  In the worst case, you restart the move at the last
checkpoint and just waste some time copying data that was already copied
but not checkpointed.

Thanks again. In those other utilities, since the logs get written to a plain text file of the user's choosing, and partition moving should be offline, anyhow, it would be reasonable to expect the user to provide a safe location to stash a checkpoint file.

And, of course, if the user chooses to manipulate the file and/or forego it altogether, their funeral.
Not sure if it's a feature worth requesting, but it would keep people like me from bothering the list with these Hail Mary support requests, so probably be net-cost negative.