In addition to the normal feed that wordpress produces, i need to publish an additional feed of posts that have a certain custom field value.
I'm a bit lost on where to start. Any tip?
Something like this will add a /feed/special to your site.
After putting it in a plugin or your theme's functions.php or what have you, you'll need to visit the Settings->Permalinks page at least once and save the settings, to get it to rebuild the permalink rules.