Create the world's most useless rainbow table? Crack passwords of whatever website still uses Sha256 to hash their passwords, and in an oddly specific turn of events uses double round at that? (disclaimer: I'm not sure if Bitcoin ASICs actually do double rounds on the hardware level, maybe you could use it for other obsolete insecure databases as well).
See they don't even do that. They don't complete the double sha and don't return the hash at the end, only nonces that meet targets. So they really are useless.
If you did a bit of coding could you get it to return the hash? Or do any other applications operate in a similar way to Bitcoin mining? I feel that they should be able to computer at least something else.