I think what we really want is to implement a blogging API that’s already supported by those tools (and a lot of other tools and services, like Flickr) as an add-on to MODx. We’ll need to do some research on exactly which API to select, and how that API would "fit" with MODx vs. a dedicated blogging tool, but I think this is a great idea that deserves a feature request in our tracker.
Is support available or planned to support Ecto, Mars Edit and similar client tools for posting content to ModX sites?