> You fixed something from the immediately-buildable tarball that you'd rebuilt and tested

That entirely depends on whether the necessary libraries and headers are installed on the system you're using.

> even something that didn't need a rebuild (config, doc, interpreted source), and sent the change, perhaps using M-x diff-backup

Unless someone uses emacs and gnus or something similar, email clients could end up mangling the diff unless it was sent as an attachment (which would make reviewing the code and posting an inline response more difficult).