Olly,
Wow. Just as well you did take them off and have a look!
I was speaking to my mechanic last week and we went through the theoretical causes that cause the cracks that we've discussed on here, and actually came to the conclusion that none of them should cause the cracks unless the part was incredibly weak.
1) Seized bolts. So what.. the centre shouldn't move anyway so a seized bolt shouldn't put any more load on the wishbone.
2) Incorrect tool used to swap bushes. Well yes if somebody used a hammer and a socket to bash them in yes, but done correctly with the wishbone supported there is no load going through the arm.
3) Tightening up bolts whilst unloaded. The Maserati service manual shows you full assembly whilst the subframe is off the car... so impossible to load them into the correct position going by "the book". They'd have to be a long way out to cause this and the chances are you wouldn't be able to connect the suspension components if they were that far out.
So the only thing I can think of is old age and metal fatigue is the prime culprit... or unless there is something causing the bush rubber to expand over time causing an excess of pressure on the wishbone?.. but generally bushes wear and become sloppy?.. Hmm..
Mark