Rcpilet:
A balsa wing isn't always sheeted on the whole wing
Why would you have to fully sheet a foam wing? Couldn't you sheet the way you would on a built up wing? That would mean to cut cap strips and leave the "rib bays" open- with exception to LE sheeting to the spar, and TE sheeting?
Of course the foam would still be there, but between the covering and the sheeting with mock cap strips, there would still be a lot of strength, fooling the normal eye to a regularly stick built wing. Wouldn't there?