rethinking the mechanism...

How about this...
http://www.home.earthlink.net/~capri26/#tiller

Ok, I'm not drawing threads! It's a greatbigpaininthebut.
But you get the idea?

Both end pieces (forks) have threaded rod firmly affixed.
They are joined in the center by a long coupler (my control handle).

I could probably make that center handle by screwing a couple of
nuts onto a piece of all-thread and brazing a tube between them.
However long that handle is determines total throw range.
But no reason it couldn't be a foot long if desired.

Total parts count:

Two pieces of threaded rod, maybe identical, maybe one a bit longer?
Two identical threaded rod end pieces - Lock tight or pinned?
The center barrel a.k.a. control handle.
A fork for the tiller pin.
A fork for the AP mount.
And a couple of shear pins or bolts to hold the forks on...

Like I said, I had it over constrained.
You busted that loose.
Thanks, Ian.

So what kind of boat do you have?
And where do you sail?

Richard