The HoA is owned and run by the builder, actually, so getting them involved will not be helpful.
KeenReader, the neighbor isn't at all upset with me or my family over this, but she was rarin' to go against the builder - sorry if my post wasn't clear on that. She actually even "boo'd" my family his Halloween, so things are good between us - we both agree that the builder messed up here, though.
She never did put up the banner but I noticed she stopped trying to grow things in her yard so maybe she gave up. It will probably resurface again this spring.