I have the same problem on my Morgan 323. The prop is in an aperature and then the aperature frame runs right into the rudder shoe and full skeg so its like it is a full keel design for reversing. Needless to say it does not back up straight for the life of me. As soon as put it in reverse the stern just walks to port and the wheel does absolutely nothing.
I have learned to live with it though and just take extra time to plan any docking meneavure before hand. For instance if I want to back my boat into a slip instead of lining it up straight and then hitting reverse, I point the bow 45 degrees to the left so when I reverse it the prop walk lines the boat into the slip. Then to fine tune the angle I do short bursts of switching it into forward gear and then back into reverse as again the wheel does nothing.
Overtime you will learn how your boat responds to this type of thing so you will know exactly how to line it up, how far to position it and at which point the boat rotates and which point it starts to actually move backwards. Couple that with the wind and tide.....
It is just something you have to accept and do your best with it. The most annoying thing I find is not the fact that my boat doesn't back up but the fact that other people on both spade rudder light disp sailboats and power boats, don't understand what we go through. I was going to kill my friend the other day as he was on the dock yelling instructions to me as I was trying to back my boat into a slip. "Just turn the wheel hard over" he keeps yelling. I honestly wanted to punch him because he is yelling like I'm an idiot mean while his stink pot as a bow thruster!