How to: Have WP send trackbacks for a URL stored in a Custom Field?

I’ve hacked up my WP for a linkblog. Instead of using a local permalink, it links only to another URL. This is the case both on the site and in rss feeds.

I’m using a Custom Field to store the value of the external URL. What do I need to modify in WP to ensure that the URL in the Custom Field (call it “sourcelink”) is sent a trackback/pingback?