I've settled on a hopefully credible way to sign my posts without needing to worry about how word-wrapping will mess up the GPG signature. Before signing my posts, I will remove spaces, tabs, newlines, and any other non-printable characters before signing.
That means that the GPG signature of my posts will be invalid as-is (since the text of the posting itself retains the whitespace and non-printables). That means you will have to remove the whitespace/non-printables yourself before running through the GPG verification to see that it is a valid signature. This is how to do it:
put the plain text into PlainTextFile, and the signature (including beginning and end lines) into SignatureFile, and use this command:
tr -cd [:graph:] <PlainTextFile | gpg --verify SignatureFile -
Don't forget the single hyphen at the end of the line, which has a space before it.