I've had a quick scan through ESB and all shots of R2 standing on two legs with Mr.Baker acting are shot in a way to hide the 'socks'.
He's either shot from above middle, his feet has a tree stump in foreground or whilst watching the x-wing sink has dry-ice disguising the copex.
The only shot you see the signs of Kenny's legs are at the Finale of the movie.
ANH has quite a few scenes, dunno bout ROTJ.
I'm gonna do a wee mock-up to see how the scene goes.
J