So, after that long-winded explanation of things... Here are a few working bullet points for you:

• A FeedMagick pass-through feed filter — (source)
• My del.icio.us links
• My del.icio.us links, passed through
• A FeedMagick filter on dc:subject — (source)
• My del.icio.us links, filtered on dc:subject="ajax"

Granted, this is gratuitous because there is already a separate feed for my ajax-tagged links. But, a lot of sites' feeds aren't available with such granularity.

And, finally... Yeah, this is a work in progress. And my first grumble, where ignorance fails, is that damn rdf:li sequence in RSS 1.0. Right now, this filter only knows about items in the parsing stream—so all the rdf:li's stay in place. Suggestions welcome!

On the other hand, it works pretty well for Atom and RSS 2.0 so far. It's just that RSS 1.0 feeds at del.icio.us are one of the best public implementations of feeds with dc:subject in the items.