Does "new com webex" work for you in Chrome? It won't offer anything but a Google search for me. On Firefox it'll either offer to switch tab to here if it's still open or it'll find this in history.

I agree Chrome's site search thing is awesome, but how did you teach it to use the new HN one?

EDIT: Chrome settings -> manage search engines. Very impressed with the list of those it's stored automatically. Point stands that it doesn't seem to relearn though when one changes.