Maybe the backup file could go in /boot ?? That won't help you with booting though.
I think that using a rescue CD is the only way to finish the reshape if you ever need to shutdown, which you would need to do to move the backup file.
The rescue CD would need to have access to the backup file, and you would assemble the array with
Then just wait for it to complete. It should go faster than with the backup file on NFS, but it still won't go nearly as fast as a resync.