I wish I knew how to scan in a diagram, as I know a way how to make it work. But describing the schematic for said solution is overly complex to explain via keyboard.
That said, have you contacted the company & asked about this conundrum, & the reasons whey they specify that the AP be mounted thusly? Is it due to a sensor in the unit, for detecting rudder angle, or? And you might also inquire as to why this is important, given that it's going to be in a sailing vessel, & thus heeled over most of the time.
Also, I'm having a bit of trouble picturing things in my head
at the moment, but is there a way that an arm attached to the rudder post, which is bent so that it's in the horizontal plane, work?
It'd be worth it too, to do a Google
search for pictures of Auto Pilot installations, to perhaps get some ideas, & or solutions.
BTW, what's the make & model on the AP in question? It'll help us with finding you a solution. Ditto if we know the make & model of the boat, plus if you could post any pics of where (as in the space/area) you plan to install it.