Yay! Today DaveWiner gave a shout out for this: RFC: ?MetaWeblog API. It's exactly, precisely, what I wanted. Not perfect, but allowing arbitrary metadata, with a preference toward RSS attributes, is pretty much what I wanted. It's basically what I was going to try to do in a hackish workaround way with in-blog-entry directives processed by a WeblogMulticaster.
Oh, and in other news, there's a quick and dirty implementation of the XmlRpcFilterService for Win32 in the wiki now. More work to do...
(Oh again, the content of this entry is a test of the pipeline hardcoded into the Win32 service. Not perfect yet, but getting interesting at least.)