I don't know if this is even possible given WordPress's XML-RPC, but is there a way to get MarsEdit to fetch a catalog of all the images that are currently uploaded on the server? I've been using the WordPress online interface for a month or so, and there's a bunch of images that are already uploaded that I'd love to access from within MarsEdit (since I use them often).
You know - I can see how the image Catalog being limited to only new stuff is really limiting for existing blogs. I don't think there's any mechanism to "import" the existing stuff but I will make a point of looking into this. It would be great if there was someway to add already-uploaded stuff.
Thanks! Here's hoping there's some applicable XML-RPC hook (although like you I doubt it).
If there isn't a hook, you might consider having an "import from URL" option where the user enters a URL that MarsEdit could troll through looking for images. Even if I could just drag URLs into the Catalog and have MarsEdit recognize images that were already uploaded to my blog and catalog them appropriately would be nice (if labor-intensive for me).