You'll need to do a little work to implement this, but it's not too complicated.

Using either Chrome's built-in "inspect element" feature, or the similar functionality provided by Firebug for Firefox, inspect a fashboard post by the blog you want to make a favourite. You should see a class called something like "post_tumblelog_9f9b05bc6d86a119a5f73c15523c270b". Put all of those (with "li." before each one) in a comma-separated list in the style settings below.