Changing RSS Item Links

I’m working on implementing a Daring Fireball-style Linked List. I have my Linked List posts in a separate catagory (4). And the posts in that category have a custom field called “link” with the external link. How do I modify my syndicated feeds to change the link on all posts in catagory 4 to the link specified in the custom field? (I’ve already customized the site to display them correctly).

I think the following code would work, but where can I put it?

if ( in_category(4) ) echo(get_post_meta($post->ID, 'link', true)); //return the custom link
else ; // normal permalink