If anyone knows where to find a legitimate MIPS version of Photon, I’d be interested in starting a decomp project.

I don't think a MIPS version ever existed. The only public releases that ran on MIPS (6.4 and pre-SP1 6.5) only bundled a handful of Photon-related binaries, which if my memory serves were just input drivers.

There were x86 and more or less complete ARMv7, PowerPC, and SuperH ports.