My Understanding is that there is no equivalent to MR for rF2. I would hope that once it has been developed for AC it will be moved over. However, I suspect that some of the issues may be the data that the program needs to have access to. But rF2 is pretty much and open book so this may not be an issue at all.
It may be more that there is not nearly as much online pickup racing happening in rF2 compared to AC so there is no reason to have an MR like rating system.