It just might work better(less binding and easier to 'find' and operate) if the slider is mounted under the deck sticking down vertically beside the fixable peg. Some possible slight rope sag for you(unsupported for 12in or so)but minimal for you son.

Some other future idea might be to just use a camlock as the footpeg on the slider. Wavesport footpegs work like camlocks but to the slot in the rail, but this case is applying to the carriage and not the rail, and horizontally. So the slider slides on the rail and webbing and the camlock clamps the webbing only.

The other positive with your lockout, is that it will be much easier to set the footpegs and whatever actuator you actually decide to use as you have a locked in zero point to really help as you're scrabbling around w/ the arms reaching way far in the yak.

Another addition, might be to make the lock 'knob' or whatever on the cockpit end be a on or off latch rather than a slider. Then in addition to the lockout, say 12 in or so in front of the rudder post, the line(s) feed thru a sleeve or sleeves that will squeeze and the lockout cable attaches to this clamp. Many variations.

Anyway the idea is that the lockout not only locks at centrline but also clamps the rudder at any angle than you want, like for those long crossings. Or if the lockout was push pull or a push shaft, the lockout pin could have a bike brake pad attached to do the same thing the pin only fits the lockout, but the brake works everywhere else if you want.

-mick