Limiting placements in Pacific will prevent USA to rush Japan but Won't prevent USA to go only Atlantic since Pacific islands are worthless realistically, USA would only spend land units for to defend the mainland, USA would never buy any single ship in the Pacific.
The main issue is not only worhless Pacific islands, also naval units are more expensive than land units. Without producion penalty or making pacific islands unrealistically valuable USA will always go for Atlantic. Invading France and trying to knock Germany out of war will always be more valuable than trying to save Australia.
I have convinced myself, production penalty would be only solution and the better one than making valuable pacific islands.