F: "Only problem I see with manipulandum is no one would know what you mean."

I agree with Faldage. The word is too obscure.

JM: 'Words like missile and projectile are too broad'
They're too specific.

JM: 'So a word that means, "A manipulandum that is the focus of, or central to, play in a game"'
Ah ... I misunderstood. I don't think manipulandum is something that is central to the play in a game. It's anything that one manipulates. I'm stumped.