Regarding your question, I think the difficulty of creating such an app would depend on some unknown factors. The internal HDD is per-console encrypted, and while we already know how to retrieve those keys and do the decryption/encryption, it adds some complexity.
The PS3's built-in format utility targets the internal drive and it may or may not be written in such a way that it could be patched to target USB instead. If you had to write your own utility, you'd need to figure out the partition structure, which as far as I know, nobody has done yet.
I tried something similar back before the PS3 was hacked. I was trying to find a way to transfer >4GB videos from the PS3 to a USB drive, so I thought the PS3's own format would be an obvious choice. Unfortunately, it didn't work -- the PS3 wouldn't mount the USB drive when in that format. If that has secretly changed in one of the updates, that is pretty interesting.