On the Dutch forum we have been discussing a solution for wishbones with haircracks (Boomerang's idea). Not meant as a permanent, long term solution but aimed at keeping our cars on the road, i.e. to be able to drive our cars without having to worry about fatal incidents as a result of a fully sheared wishbone.
The idea is simple: fabricate a round shaped, steel sleeve that covers the mounting part of the arm (the part with cracks or which may develop future cracks) and weld some flaps to this sleeve, both to the top and to the bottom of the arm and make sure that the flaps stick out a bit to the sides of the arms, so that to the sides of each flap a hole can be drilled in order to be able to bolt the flaps together (with the arm in between). Alternatively, the sleeve can be secured by drilling a hole through the arm itself but I don't know whether that would be a good idea from a safety perspective.
The steel sleeve + flaps would obviously need a coating or they would have to be fabricated from a material that is less prone to corrosion.
The sleeve itself could be as wide as the part of mounting hole that is "narrow". See arrows on the picture
To illustrate the position of the flaps, see below picture (apologies for the crappy picture). The red dots indicate the position of the bolts which would have to keep the unit in place.
Perhaps it would be an idea to manufacture/machine something out of 1 piece/sheet of (stainless) steel with a snug fit that could be pressed/slidden over the mounting hole from the side?