A couple of points abotu this

the seat/seat back relationship is important, and nearly as important to the balance of the kayak as the seat position. any design should start with that in mind.

I don't really like systems that require me to undo the spray skirt to fiddle around inside. something along the lines of "when the going gets tough, the tough get paddleing, the soon to be wet start fiddling around with the settigns"

the ball berrings idea would A)seem to work fine untill you get grit into the cockpit, and b) concentrate teh load on some small areas of teh kayak. I would suggest using runners attached to the kayak that match runners on the seat. tounge and groove may work. keep drainage and grit in mind when designing these.

if you attach the pegs to the seat and the seat to the kayak, changeing the position of the seat automagically changes the postion of the pegs, however that brings up my next thought

normally you grip onto your boat because both the pegs and seat are attached. however if the fit to the boat is sloppy, you may inadvertantly create an ejection seat. Nothing ruins your roll faster than having you boat come apart on you (that kinda happened to me once)

side note, kayak ejection seat, you can use those estees solid fuel rocket motors as propultion, hmmm, a C motor produces about 3N for 4 sec, at about 5N==1Lb, you would only need 416 2/3 C motors to produce 250 Lbs of force, they don't work when wet, you would need a car battery to ignite them all, and you'd probably set your kayak on fire testing it, but it is technically possilble.

here's an idea. attach the pegs to the seat (via a rope, cable or rod) since the likely hood of your legs growing while at sea are negligable, you can assume that the placement will be somewhat static and will be correct when setting out. since load on teh pegs is forward, and teh load on the seat from the pegs is backward, but the load on the seat to teh kayak is backward (imagine if you were to dump some vasoline into the cockpit of your boat and started paddeling which way thing would start sliding, but I don't want to you to ever tell me what you are thinking about when sitting in a lubed up boat)

how about if you were to attach some rails to capture teh seat from the sides. you can run a dynel or spectra ( sailboat halyard style ) rope from the seat to a jam/cam cleat mounted on or near the front bulkhead, (say spring loaded with a foot release)

another option is to get a couple of 2 ltr drink bladders (e.g the camel pack type) attach them together with a hand pump in the middle, one of the bulb type that you find in outboard motor fuel systems. You could fill them half full of water, have them mouned at etither end of the boat and you'd be able to ajust the trim of the boat without moving.

another option (that is even wierder) is to get the powered seat ajustment off of a boo-ick. ( on my dad's car, depending on which key/radio is used to unlock the car it will change the seat position, height, mirror postions and radio station to the settings of that driver)

please excuse my ramblings, I'm being medicated and am unusually light-headed.