In the discussion about rudder foils it was mentioned that the foils might cheat the boat into thinking or acting like it had a longer waterline length.
I was just looking at the box rule for F14 and the rudders and any foils or fittings assciated with them are not measured into length.
Does this mean one could creat a longer waterline with such an appendage specifically?

I don't see why not. In some classes (18 Sq Metre comes to mind) the rudder IS measured as part of the hull length, if it is wider than 3"; I'm not sure whether that's at the waterline only, or at any point. Also, the I14 skiffs are using foils, not to increase waterline per se, but to "trick" the water into flowing as if the hull were longer. It's working extremely well.

I saw that too. In theory it's an advantage. In practice it get's in the way of the rudders and they can be fraigle.

I had a system that attached to the rudder pin and extended the hull to about 25% of the rudder width on my 14. It seemed to work very well in light air and one side fell off in heay air. It seem to reduce turbulance at the stern but it was not worth fixing because at that time I made bigger gains learning the boat.