>> Although...if COPY can handle the now() function, it should be able
>> to handle the current_user/current_timestamp without too much overhead.
Eh? COPY most assuredly does not handle the now() function, nor any
other function call.
It will accept 'now' --- no parens --- as an input for datetime
datatypes, but that's because those datatypes do, not because COPY
is doing anything special.
IMHO the entire point of COPY is to load bulk data as quickly as
possible ... if you want any convenience processing done for you,
you should be using INSERT not COPY.
regards, tom lane