Hmm, how about making Metal pinball posts?
T nuts, shooter housings parts with custom logos?
They are expensive and a lot of times there are hard to find specific sizes.
Pinball league ID tags?
Tournament prizes?
Glass rails and guides?

Seems like the chess set idea is a good one too...

I would image The amounts you’d sell would not make it worth while but only you could answers that question.

A sister company I work for has patents on ship flanges and clients include Lockheed Martin and
GE. I can’t help with specific leads but my advice would be to network with companies that make parts for companies that have huge government contracts.