Most of the recovery software destroy the format and file system to the file that was coded in like UTF or Unicode.So there is more likely the chances of file being destroyed while the recovery.
You can, however, dd whatever's on the disk to a program to look for memory that looks like private keys. Assuming they were unencrypted. Dooglus has posted a program that does just this. If I were trying to recover keys from a disk, the first thing I'd do is make a copy of the entire memory on the disk before using it any further, that way if there is an unencrypted private key on the disk, it won't get written over.