Em dashes should always be treated as a wordbreak, though; is that much software really broken regarding them? Just as a quick test with what I have installed on this computer, grep, Emacs (in fundamental-mode and text-mode) and LibreOffice Writer all recognise it as a word boundary.
This came up in an editing and typesetting discussion one day. I've been convinced ever since that there should be at least a small space around em dashes.