Where under OS 6.x is the backup file for a reshape kept?

Where under OS 6.x is the backup file for a reshape kept?

Background:
 
ReadyNAS Pro Business running OS 6.2.4 had just started a (0.3% done) Raid 5 to Raid 6 reshape when the UPS it was plugged into failed unexpectedly.  After booting back up, the data partition wouldn't mount. Investigation over SSH found mdadm couldn't/wouldn't assemble the array without a backup file for reshape.

So far:
 
The array successfully assembled using an empty backup file using --backup-file=fubar --invalid-backup --freeze-reshape but still wouldn't mount.  Btrfs recover successfully found ~33% of the data (not already available in other backups) stored on the array.

Next Step:
 
Restart the reshape (if possible) in the hopes the array can rebuild enough of the missing data to mount, or retrieve significantly more data through Btrfs recover.  But I want to use the original backup (instead of an invalid/empty one) if at all possible. I've been unable to find any likely candidates for that file.