Brian, you've highlighted a real gap in the market here. 54mm/pseudo54mm spare heads abound, for example, I've a box full of 'em.
If somebody put out some spare heads in 75mm & above they'd do well. I am sure there may be producers reading this who may have some they felt weren't quite good enough to use on a figure, but could get away with in the context of 'spare heads' even ones minus hair etc. Or even existing kit heads in a pack of 4 or 5? In resin, they'd be easy to convert.
Mike highlights compatibility issues, and that is a very valid point though. Some Post Militaire 90mm is nearer 110mm.