the BASIC correct pivot line if using "wingerons" would be 27% MAC line... just as that is the correct pivot line for a "flying stabilizer." You are doig an aerodynamicly ballanced control surface... and its the whole wing.

I've seen it done... the throw was fairly small but it still had a FAST roll rate. Just +/- 3 to 5 deg at max throw. (hard to guestimate that small an angle by eye)