I would probably try to take an RSS feed, but set it to list all of the posts, instead of the last 10 or whatever the default is. You might run into memory errors or php timeouts if it takes too long to generate all of the content.
I'm trying to do the same and I've set the blog to list 1000 posts (way more than all of them) and to consider 1000 of them current and it doesn't seem to work. I still only get the first 25 or so posts in the XML file. My feed is at: