In closed loop (<60% TPS) - the WRX controls fuel fine off of its front sensor and is alread based on AFR. In open loop the WRX runs fuel based on layering of load maps, it does not target an AFR since it can't even tell what the AFR is below about 11.24-12.00 and most of the fuel maps are set at levels that will usually leave you close to 10.0:1.
I assume you are talking about continuous AFR based closed loop fuel control. Even with the proper sensors the stock ECU is not set up for this - you would need a replacement ECU like Autronic to provide continuous closed loop fuel control. This WBO2 converter would be the right unit to do that with. I am not aware of any piggy back solutions that would take over fuel or even currently accept an aftermarket AFR signal.
Both WBO2 sensors that are available recommend against putting them pre-turbo (pressure issues). I don't know if this converter can adjust for a pre-turbo location. This kit would work well in a cat delete pipe or towards the bottom of the DP if someone had the third cat.