The experimental ebuild for Gentoo worked like a charm: I am typing this on Bilbo. The editor seems to Just Work (tm), the previews are functioning (both rich text and the website preview) and the account creation worked out of the box.
The only thing that took some time to figure out is the fact that the auto-config option did not configure my site. In fact it started looking for a xmlrpc.php file to talk to, which results in a 404 as Joomla! does not have a file but rather a folder with that name. So after setting it to the correct location and switching to the metaWeblog API in the Advanced settings I hit 'Fetch blog ID' and voila, we are up and running.
Well, kind of: the plugin I use to provide the blog API on the site is incomplete. I have been tweaking it for months now, adding missing functions as I went. So when I wanted to select a category, I got an error stating that the XMLRPC function was not found.
After enabling the debug info on my server, I saw that Bilbo tried to access metaWeblog.getCategories - which is unimplemented in the original version of the plugin. So after adding the missing function, everything came to live and here we are...
Now if you excuse me, I have a virtual machine to dispose of as I can retire Windows Live Writer now ^^