I really dislike any design that insists that servicing/repair should not be possible, since that is one thing that separates toys from models. On some of my smaller ones, where an internal linkage was not going to be advisable, I have gone above deck, and painted the bits bright red. This invariably renders them invisible, something to do with the fact that something so blatantly inappropriate really must be on the real thing. If the top of the rudder tube would be above the waterline with the tiller below deck, a hatch is the way to go, either disguised into invisibility, or made so obvious that it "must have been there" on the real thing. If it is anticipated that access is going to be very occasional, and that part of the deck is liable to get wet, then a hatch, possibly of a sacrificial nature, sealed down with water resistant, but easily frangible adhesive, will do the job.