Assuming you're wanting complete redundancy without spending the money on something like a hydraulic ram (which would be useful for an autopilot, btw...) and you want to use a tiller since you've got a fitting for a tiller...
I would think that the shaft would have to be extended to an above deck position to be used with a tiller. That's most efficient.
We can be really "Rube Goldberg" here... Lets think like big, old, ships...the is the type of system on the HMS Rose
aka the Surprise
in the movie Master and Commander
...you have room to put in a below decks tiller with line running from each side of the tiller to some sheaves (turning blocks) in your stateroom that would lead to some sort of emergency old-fashioned barrel type wheel on deck. The lines are wrapped several times around the barrel/wheel and you turn it to move the tiller as desired. If you need more leverage than your 5 ft (or shorter) tiller can provide, you can add purchase with blocks of course but that means even more turns on the wheel to have any turn on the tiller. That whole tiller below deck and barrel wheel above deck used to be pretty common.
If you have absolutely no way of bringing your tiller up directly onto the deck and, realistically, I can't image that you'll have a way of making a whole separate barrel wheel for the system I described...you still could use an emergency tiller below deck with two lines and strategically placed blocks to bring the lines to a position that they can be pulled on to move the tiller back and forth. It would be very difficult (I'd think) to steer like this and it would be much more likely that someone would put push-pull lines on the rudder itself in an emergency where the only way one could steer would be using lines.
Back out of the "Rube" mode and into reality: Get an extension fabricated and put a tiller head (whatever it's called...the square lug) up in the cockpit
Pic of a "barrel wheel" type helm on the Surprise
. On the LINK to bigger pic
you can more clearly see the wraps of line around the barrel wheel.