PeopleAggregator supports a reasonable amount of the metaWeblog API (scroll down for the metaWeblog.* and blogger.* methods) as well as RSD (example), although it seems that this isn't quite enough for most editing tools. I think I must have done something wrong with the RSD, too, as nothing seems to be able to figure out my blog settings from my profile URL and RSD document.
Here's a rundown of what I've tested so far:
Qumana - claims to be unable to autodetect blog settings but successfully parses the XML-RPC endpoint out of the RSD document. Retrieves list of blogs and publishes a posts without any trouble. Fails to retrieve posts from the server.
Working on getting them to work properly at the moment... stay tuned for more updates.