So I downloaded the SDK for WL Writer and took a quick look. Basically, there's two types of extensions you can build:
Given that they are trying to support "every blogging service out there", I'm surprised there's not a way to build a plugable blogging service. WL Writer only allows you to customize the content of the post via plugins. Customizing the metadata (i.e. categories) is right out. I realize it's the hip thing to put Technorati tags right in your post content, but Technorati also picks up category information which dasBlog already has great support for. What I'd really like is something that acts like del.icio.us' new post form, where you can free type in your categories, it highlights words as you type and it shows you a list of all your tags so you can click on them.
One other minor note - WL Writer does a good job for inserting hyperlinks. When you select a word, often the whitespace that follows it is also selected. Some HTML editors will insert the hyperlink over the whole selection - inlcuding the whitespace which makes no sense. WL Writer gets it right and excludes any trailing whitespace from the hyperlink. Cool!