The principle of the adjuster is perfectly sound - the issue here would seem to be the material strength related to our wishbone designs and the loads seen.
My calculation is a very simplified analysis of the situation as it does not consider the actual wishbone swept geometry.For example if the the roadster wishbones travel through 25 degrees and the locost only travel through 20 degrees then the difference in loads will be much greater.
One other possible thought for construction is that by boring the bosses before welding them - the holes may become oval.
Without doing calcs I couldn't say for sure - but I suspect a set in mild steel would do the job. I plan to make my own from EN24T - not becuase of the strength - but due to its good performance in cyclic loading conditions - and beause I have some lying around
The important area to be concerened about is the head and the surface area as this is now taking the load. Put another way we know the thread will take the load as it does in a standard wishbone. You need to be careful with the transition between the shank of the adjuster of the head as any sharp edges will be stress raisers that could start a failure. Ideally you will be looking for something like a 2mm+ radius on both the adjuster and the matching boss.
I hope that is some help - sorry I can't be more specific with a definite answeras to design and material.