In reply to:
I think relying on rel-webmention is the right thing to do long term. If one advertises that one accepts mentions, but still doesn't do it, then something is wrong and since it's still early on we should try to fix the cases where rel-webmention is advertised wrongly rather than try to build workarounds. If we are to build workarounds, then we should first document why rel-webmention doesn't work and why we can't fix it so that it works.
+1 on bringing it up!
(I hope to contribute by making it very easy to set up such a mechanism using my Webmention endpoint. I have one set up at http://voxpelli.com/mentions/)