It is, but why would you want the diags partition of a jailbroken 5.1? The jailbreak lives in the "main" partition
I think I know where you're trying to go, but the images are really only there for recovery purposes if something seriously fucks up. Plus, actually "flashing" the images is not exactly trivial either. So no, a full-fledged image of 300 MB is not a good substitute for a 20 kB jailbreak
