To what purpose? The current ailerons are incredibly effective to below stall speed and the RV-12 already cruises "near " its LSA limits. Seems like a lot of complexity for minimal if any useful benefit.
Ooopps - just saw that you are EAB.....go for it if you like.
After sealing up the wing roots (as designed), It seems at least something (speed) could be gained by gap seals. Maybe not, but as a true EAB "experimenter", I'd steal for a knot - and kill for three
Speed is like money...Mo is Bettah!
RV elevators and rudders are simple hinged control surfaces where the aerodynamic influences of something like gap seals would be unlikely to ever have any negative effect on flight / handling characteristics.
The same can not be said for the ailerons (including the flapperons on the RV-12).
Aileron performance is actively influenced (by design) by the shape and the way that air flows around them. Making changes that influence that flow will definitely put you in the experimenter category.
Two guys sealed the flaperon gap on their RV-12 with some type of tape (mylar?) and claimed it improved climb performance. They had the plane at OSH about 5 years ago and even showed the mod to Van himself. One of the two builders still posts regularly on this forum. Perhaps he will comment and let us know if the tape has held up or if it was removed.