UNA now supports the metaweblog api so that articles can be created and managed using WLW, please note you need to use it together with MetaPost (the community edition is free), you also can find MeatPost on snowcovered by searching for "MetaPost".
The key of UNA in MeatPost is "Ultra News Article", to connect to UNA in Windows Live Writer, just follow these simple steps:
1, "Add blog account" from WLW menu.
2, choose "Other Blog Service" in the 1st step.
3, Enter your site url, and user credential.
4, choose "Metatweblog API" in the follow step. and enter "http://yourdomain.com/metapost.ashx?key=ultra news article" as the remote posting url.
5, select a UNA instance on your website.