In theory this might seem logical, but in fact, where the mainsheet sheets to the boat has no effect on weatherhelm. If you move your sheeting point to a traveller forward of the companionway, you will likely bend your boom in a big wind.
Weatherhelm is the difference between the center of lateral resistance on the keel, and the center of effort from the sails. The CE will be aft of a vertical line drawn from the CLR and that difference is called the lead (rhymes with seed). The more lead--the more the CE is aft of the CLR--the more weatherhelm.
The only way to reduce weatherhelm is to move the CE point forward. When you reef the main you do just that (the smaller the triangle, the more the CE moves forward). If you move it forward of the CLR you will have lee helm (drop your main and sail on your genny and you will see what I mean).
To move it forward you need to either move the mast forward, which you can do by adjusting your rigging (it will help a tiny bit but on the B29, not enough to be really noticeable). You can move the jib forward using a bowsprit, you can shorten the boom and reduce the size of the main, or you can sail with a reef most of the time. The problem with the B29 is the giant main that was encouraged under the old CCA racing rules. Making the boat stiffer can also help a little bit so you could add ballast into the lowest point in the bilge.
Whatever you do, don't mess with the rudder. Changing the shape of the rudder does not affect weatherhelm, it will only ruin the sailing qualities of the boat.