> (Of course, for the combined feed this would be problematic.)
Not so; unlike the HTML <base> element which applies to a document, the xml:base attribute is applied to an element and its descendants. The typical pattern (as shown in the RFC 4287 example) is to put it on each entry’s <content>. In your markup, you’ll end up with each entry having its URL in three places:
Excellent! I'll look into actually implementing this before making further comments, since I'm sure I'll find out such things as I do :P
Edit: the package I'm using to generate the feeds does not support that attribute yet, so it'll have to wait a bit for my PR to hopefully be accepted: https://github.com/jpmonette/feed/pull/158