The short version: I ran across some modifications of the Press This bookmarklet that claims to make it automatically add a custom field for a link URL. I love the idea. But it doesn't work for me. The code for the modified bookmarklet is here.
Basically, as far as I can tell, it just pulls the URL into a custom made box on the bottom of the original (around line 550), pulls the value of that custom field after submission (around line 50), and then uses add_post_meta() to save that URL in the chosen custom field (around line 80).
But somewhere along the way it fails. The posts never gets anything added to the custom field.
Any help is greatly appreciated.