psql -h localhost postgres -c "copy (SELECT * FROM a WHERE time < now()) to stdout " | psql -h localhost postgres -c "copy b from stdin"
The first question at hand is whether the source psql command will provoke an EOF (which is the only thing that will stop the copy-from) even though the complete contents of the copy-to have not yet been sent.
The second question is whether, even if it does send EOF, the second command will be allowed to see that EOF and complete its command. Pipeline failure mode might impact this (from bash experience).
Unfortunately I do not know the answers to those questions.
The source code might be of some help on the first.
Another option is to replace the first psql process with custom program that will send data to stdout and in the middle of doing so die with a non-zero exit code. That should be a workable simulation of the copy to command and evaluation of the target can be done to see what behavior is exhibitied.