A little barnyard engineering follows. It may be my concerns about wing twist are a little overblown.
Basically, through various approaches I come up with calculated loads leading to a maximum of some 1200 pound-inches of torque on the rear spar over a 9 foot long 18 inch chord flap. If that is spread over 4 hinges, then there is 300 pound-inches for each hinge (figuring center of pressure at 1/2 chord).
So I was wondering what my frail-looking ribs could handle. I decided to put a piece of 3/4 ply in a vise and stick the rear spar opening of a partially completed rib, and then apply force at the front spar opening to see what would break.
At 9 pounds 15 ounces, my plywood-in-a vice failed.
I reattached the plywood and got up to 10 pounds before the rib started to twist (in a wing, twist is prevented by cross taping).
So, one rib with only half of the gussets on it can hold a torque of 310 pound-inches on the spar without failing. (31 inch center X 10 pounds).
Then there is the fact that at 14" spacing for each rib, 9 feet of spar is supported by 7.7 ribs. 300 pound-inches times 7.7 ribs is 2100 inch-pounds of demonstrated torque support.
I suppose that is why people put flaps on the 2+2 without giving it much thought--there seems to be quite a bit of reserve just in relying on ribs, not including stiffness of the spar and other reinforcing.
I still want to add a stiffening compression rib near hinge points, belt and suspenders I suppose.
Because I'm a hopeless nerd, the other thing I want to do is empirically verify the torque forces of such a flap at various speeds. I'm contemplating a set up in the back of my pickup that would allow me to measure actual forces at various highway speeds and angles.