No, there currently isnt any documentation on the libext2fs library. The library is relatively well structured internally, and so most people who have looked at it haven't had *too* much trouble figuring it out.
It would be nice to have some documentation on it, though, and I am soliciting volunteers who would be willing to do a first pass documentation on it; I'm definitely willing to work with someone who is interested in doing that sort of tech writing.
As for your question, you can absolutely tell it to use a backup superblock, just take a look at the source code for the function signature for the ext2fs_open() function. One of the arguments is "superblock", and that's the block number for the superblock. You're right that debugfs currently doesn't have a method for opening the filesystem with a backup superblock. E2fsck most certainly does have a way to do this, though, and it's documented in the man page. Try using "e2fsck -b 8193".