All North American routes from NZ can be done on 180 minute EDTO, you just lose flexibility on routing as after the Pacific Islands there’s only HNL/ITO for coverage. But NA is 777 territory anyway.
IAH would be problematic as the routing would be too far north, but not impossible. EZE is the real issue as routing that far North would simply not be economically viable.
If the 787 gets chopped back to 180EDTO (or less) then I suspect the company will simply shift some 777’s off Asian or HNL routes to pickup IAH/EZE.
So the real issue will be around capacity while Aircraft are grounded rather than EDTO. 240 EDTO is often used for route flexibility across the pacific and I think our max is 330 for EZE. Both of which the 777’s have approval for.