I couldn't get my head round this working as John wanted. It will do something but not quite what John described. I had thought of it as a possible solution.
When the conversion is made to sRGB the OoG colours will be put on the periphery of the sRGB gamut.
When the difference is taken the result will not be the sRGB gamut subtracted from the ProPhoto Image but the subtraction will be the sRGB image plus the OoG colours using RC Intent. If Perceptual were available that would cause more non-John results.
The results might satisfy his artistic intent but I am not convinced it meets his stated intent.