Question: In particular, I don't mean mere alternate spellings like *colour*, *honour*, but words that are entirely different: using *lift* instead of *elevator*, *fridge* instead of *refrigerator* etc.

What is the common outlook on using them in plain informal speech in the US? What are the chances they won't be recognized at all? Will they be seen as pretentious? Weird? Unwelcome?

Answer: 
AFAIK *most* Americans know the British equivalents for their words, and vice versa.

Some people even use them (some Britons use the American words). It wouldn't be considered weird - an American would just assume you were British, or learnt British English. It wouldn't be considered unwelcome either, by the vast majority of Americans.
In general, I would say no, but there are some exceptions. The first time I encountered a sign in a public place saying "toilet," I felt it was quite crude. Americans would only use that word for the actual fixture, preferring restroom for such contexts.