I bounced this idea off of my other players, and we had tentative agreement. Let me know what you think:
If the USA falls, Japan gets the IPCs as normal less the 30 that come from the national objective. That puts the Japanese gain at only 17 or so IPCs, which leaves the UK and ANZAC on more even terms. So in a nutshell, national objective income disappears instead of changing hands.
Does this sound tenable as a house rule? I think it may need a bit of refinement, but the concept is justifiable.