When have people made a POW problem from encryption rather than hashing?

[jvanname]

So the kind of POW problem that I was thinking of is like this:

1. Suppose that E is an encryption function (such as 256 bit AES). Find a suitable hash k along with a 1 block string x so that E_k(x) XOR x<D (the Matyas–Meyer–Oseas hash).

2. Suppose that E is an encryption function (such as 256 bit AES). Find a suitable hash k along with a 1 block string x so that E_k(x) XOR x XOR k<D (the Miyaguchi–Preneel hash).

This way people can use encryption rather than the conventional hashes as their POW problem.

[jvanname]

I found that Skein, a cryptographic hash function uses a variant of the Matyas–Meyer–Oseas hash with Threefish as its encryption function. Skein is used as one of the POW problems in MyriadCoin.