In the RSP, MFC0, MFC2, and CFC2 were all susceptible to overwriting $zero. Some of us have tried waiting for some games to use handcoded assembly in an attempt to purposely overwrite $zero in their microcode (to throw off emulators), but so far what few occurrences there have been of this have not included using those 3 opcodes. Since it was decided to centralize the security of register $zero in the main R4300 CPU, it was decided to do so in the RSP as well.
toehead2001