One of the things that will be made considerably easier after the Leonardo rearchitecture is pluggable support for alternative approaches to editing. In a way, the RESTful approach I've taken has always made the use of external blog editors possible - I've just never tried it. Of course, I'm not sure the blog editors out there are that RESTful. They seem to largely use XML-RPC rather than just PUTing and/or POSTing.
I'm keen to try out MarsEdit, although I haven't investigated just how RESTful it can get. Part of the appeal of MarsEdit is posts like this one from Brent. I'd love it if more commercial software developers posted this kind of thing on their blogs.