Based on my skimming over FIPS 180-4 it looks like the only difference between SHA-512 and SHA-512/256 is the initial hash value and the fact that the final output is truncated to 256 bits.
My question is: if you have a library that does SHA-512 can you transform that to SHA-512/256?
Like maybe there could be some sort of magic string that, when prepended to the original message, makes the first 256 bytes of the SHA-512 hash of the original message the same as SHA-512/256? Or maybe you could xor the first x bytes of the original message with some magic string?
I'm guessing that there isn't such a way but I thought I'd ask all the same.