Apparently, the problem is that the password being passed through the pipeline includes a space--the one that appears between our hypothetical "d" and the pipe symbol itself. :)
So, for future reference, this works:
echo password|gpg --batch --yes --passphrase-fd 0 "filename"
Which, by the way, is exactly what the guide had said, but which I never caught onto because I did my initial testing in Powershell and didn't realize how picky cmd's echo command could be.