Advice re: featured-posts on feedburner

Currently my site uses the feedburner plugin and generates a single feed. What I’d like to do is selectively include or exclude posts in the already existing feedburner feed. To turn it, in other words, into a “featured” posts feed. So there really are two parts to this question:

1. What would people recommend as the best way to tag the content? Categories? Tags? A custom field? My inclination is a custom field, but I’m wondering if anyone has any thoughts on advantages/disadvantages of different approaches.
2. Harder question. What code should I be hacking up to make the feed selective? The feedburner plugin? The core library? Really, it ought to be done as my own plugin, but if so, I wonder if anyone has advice on what action it should be triggering on.

I appreciate your help.