I don't think it's possible for existing Windows code to run in Native
Client. As I understand it, NaCl verifies all x86 code to make sure
it's safe before executing it, and Wine needs to load x86 code
compiled for Windows directly, which will not be using the safe subset
of x86 that NaCl requires.

One would need a proper virtual machine to make that work, something
like qemu perhaps.

However, the pipelight project may be interested in support for PPAPI.