Often when we talk about clusters we think about geographical clusters. But, I think the original poster is right to consider occupational clusters as well.
I've been trying to find an explanation for the possible association between PD and the fungal disease histoplasmosis (which is common in the Ohio valley in the US, but rare in the UK). The fungus can be found in the faeces of some bats and ground feeding birds, such as the starling. This has, over the years, led me to looking at various publications aimed at bell ringers and pigeon racers, etc. Some magazines have obituaries, from which it is possible to estimate the prevalence of PD. I have never found any good evidence of a cluster.
Perhaps, similar sources for firefighters would be of interest.