But cats already are being twisted significantly under the rig loads. Off the top of my head, I think the T-foil would only reduce the twisting loads or maybe reverse them a little in special cases (high angles of attack on the foil).

What size foils are we talking about here. If you're talking about say 400mm * 150mm, you'd probably only get about 40kg of up/down force at an efficient angle of attack that doesn't create too much drag. If you want anymore force you're talking about foils on each rudder the equivalent of the total area under a moth (700*150 and 600*130 -ish)! They're gonna be very big, feasible but big. And I believe the foils are included in the beam measurement of the F16, so they will likely have to be L-foils, not T's, which will be even trickier because of the structural issues.

Just quickly read the rules again, I'm not sure if the centreboards or rudders are now included in the beam measurement. It says "overall beam" which I take to mean beam of the boat when taking everything into account in its normal position...can anyone clarify?

I'm need to think about these rudder foils some more...