Since drag is proportional to lift, the rudders would be prone to kicking up at speed... more so than conventional rudders.

Is the foil slapping the water's surface going to interfere with the Dotan stock operation?

Wear on the line/rudder entering the channel at an angle is an obvious concern. A steel rod bearing would help.

Wouter rightly pointed out geometry problems with the line/channel placement. These could be addressed simply by making the line ride a cam around the top axel, with the cam radius matching the bottom axel-line separation.

The "channel" through the rudder need not be parallel to rudder trailing edge.

You need some adjustment somewhere to get the line length just right. You probably need a little stretch in line line to so you can get the rudders down with tension on the line.

The hinge can only add drag, but kick-up eliminates the problem of cassettes scratching the foil, so you could possibly use a laminar flow foil section for less drag and make up for it.