Interesting variations on individual operators' Ops Manuals obviously exist here.
For example - I may operate my aircraft (including to the United States) without the restriction of needing my alternate's forecast (or actual) to remain at or above alternate planning minima once dispatched (I did need this prior to dispatch - at planning).
I can continue provided I have two airports within range (i.e. with 30' fuel remaining at touchdown) and landing is "assured" (i.e. it could be completed taking into account any forecast Wx deterioration and a plausible "single" failure or equipment such as Cat 3a to Cat 2). Makes sense that one of these two is the planned destination, obviously..., but it doesn't have to be.
So I can continue toward JFK even if it is 0/0, if (say) EWR and PHL have Cat II Wx with Cat IIIa available. As soon as landing at either of these two would involve 30' fuel on landing - then I'd have to go land, and land with >=30'. Not saying that's what I'd do (each real life scenario is different); but that's the limit allowable by my OM.
Not quite the same as requiring alternate minima at an alternate - and all FAA approved as an international operation foreign to the US.
Do US operators require their planning minima to remain at the alternate (FAA planning minima are defined slightly differently to those over here - though they achieve the same thing: a more conservative forecast required for alternates) thoughout flight? Even to a country whose national rules don't requite this (such as mine)?