How can I generate a RSS feed based on a custom WP_Query?

I have a WP_Query returning a list of posts of a particular post type and with a certain meta_value (custom field value)

Is there a way generate an RSS feed based of these results ?

You should use add_feed() function (take name of feed and callback) to add new feed, it will take care of settings up hooks and rewrite rules automagically.

Then in your callback function you can use query_posts() to override query before loading template.