I agree that having more Sterns in the mix than average might be a good idea due to access issues - - people might not be able to find EMs to practice on outside of traveling to shows.
One other option is to do something like Texas is: count your best X games, but at least Y of them must be on machines from different eras. e.g. they are doing 4EM, 4SS, 4 modern, best 8 total, but you have to have at least two in each age group. I'd definitely support something like that for PAPA. If they did the 5 groups you use above, you could require 1-1-1-1-2, i.e. two Sterns and one from each other era. Or arrange them in 3 age ranges and you need to do 2 of each, e.g. lump EM/SS into one group, the DMDs into one, with Sterns group 3.