MLH...
While this subject is way out of my ballpark...I can throw out some info for your review. Some of this is based on dusty brain cells but I think it is fairly accurate.
The BO-105 used a leading edge lead rope internal to the spar or in the general area. Unfortunately they had some fatal operational incidents until they put a steel end cap and erosion strip scarf joints covers to prevent slinging the rope out.
If your forward spar is hollow you might be able to insert/bond a weight material within this cavity. With today's products out there I sure there is a combination of weight and spacers that could be put together which will not affect structure integrity or function.
If your spar is solid or already filled with something then maybe look at a modified erosion strip installation on the leading edge. Depending on the sensitivity of the blade cg range, it could be as simple as adding a heavy erosion tape product (there are several approved types out there)...or possibly the fabrication of a replacement steel erosion strip made of a heavier product...or bond a second erosion strip...
If the addition of a tape or the modification/addition of an erosion strip is not practical...then maybe you could apply a flame spray metal product to the existing erosion strip (removed from the blade first) that would "maintain" the existing airfoil design and add mass to the leading edge.
And there always is good old fashion duct tape!
W1