That certainly sounds doable and easier/cheaper than the old EX Raker. A few thoughts (that you may have already considered):
If, like many old boats, there's excessive slop in other parts of the system (gudgeons, rudder pins, holes in castings for the rudder pins), this mod may not be real helpful in and of itself. It's hard to describe well in words, but, if there's slop elsewhere and the new slot is wide enough, it will likely be possible to slide the new bolt so far forward in relation to the pin/sister screws on which the cam rides that the cam will not have enough clearance to rotate properly. (This can occur with the modern adjustable system and the EZ Raker as well.) If adjusted with the rudder locked down, the cam will be bound and not rotate up; if adjusted with the rudder up, the bolt will be blocked before the cam is rotated all the way down. In such case if the bolt is slid back enough for proper cam rotation, the rudder will likely still trail back due to the excessive slop elsewhere causing unwanted tiller load. I guess the bottom line is that this mod cannot remove excessive slop located elsewhere in the system. While the rudder should lock/unlock better than it does now, this mod may not fully achieve the desired result without new rudder pins and/or gudgeon/casting bushings.
The length of the new bolt that will be inside the casting should not be threaded (for increased shear strength and minimization of damage to the cam).
This sounds like a lot of work, but for a great cause. Perhaps a local machine shop would contribute (free or reduced cost) services toward all of the needed mods in exchange for some public recognition?
I believe your idea has a lot of merit and would like to hear others' thoughts.....maybe you should revive this old Hobie Forum thread?: https://www.hobie.com/forums/viewtopic.php?f=14&t=63204
Good luck and please let us know how this works out!