Phil confirms the result of my little experiment:
Another Trackback oddity Handily you often dont need to know this URL because Movable Type does magic things (that Im not even going to attempt to describe) to handle Trackback for you when you link to a Trackback-enabled post on someones site. However, if you link to a post on your own site it doesnt work. But you can use the weird Trackback URL to force a link to appear on the old post you wanted to Trackback to. If you got this far youre brave, or now have a glazed look on your face. […]
Automatically discovering and sending Trackback Pings to your own ‘blog could be quite useful (although there must be a good reason why this doesn’t happen). If I knew a little Perl and had a little time, I’d probably try and patch Movabletype‘s Auto-Discovery routine to do this.