Can you run a mke2fs with the -n option, so that it doesn't actually do anything, but shows you what it would do and where it would place the superblocks???

Edit: From the manpage...

-n causes mke2fs to not actually create a filesystem, but display what it would do if it were to create a filesystem. This can be used to determine the location of the backup superblocks for a particular filesystem, so long as the mke2fs parameters that were passed when the filesystem was originally created are used again. (With the -n option added, of course!)