You probably could do that for most aircraft ailerons. I've thought about doing that on occasion, but just did the oblong hole thing and hit it with thin CA.

That Seagull Super Star 120 I have is an example of one that could really use the plywood buildup. I drilled through that and used one of those stud type control horns but the balsa was in no way strong enough for that, so I removed it and will probably go with your suggestion on that plane, for all surfaces.

I've even seen guys that removed covering around the control horn point, then carefully cut into the balsa to create a bed for a piece of ply, then epoxied it in place. That made one nice hardpoint. And looked pretty neat to boot.

CGr.