No, this should work. Besides procmail, you can pipe the message to any other program. So I wrote a Perl script and it works just fine for me (the Wiki has example scripts for other languages).
And if I have it pipe to procmail, it works for me too.
Are you putting the pipe in a file named .forward.postfix ? Also check to make sure you aren’t using the wrong linebreaks if you edit the files on Windows, and make sure there is a linebreak at the end of each line. You might want to make sure the .procmailrc file has PATH, MAILDIR, LOGFILE, SHELL, etc set.