I just stumbled upon
Ebook Glue, which promises "[...] to provide a stellar reading experience for text-heavy content."
I tested it with today's (1/14/13)
Slate Articles feed. The generated .epub and .mobi files, which you can download
here, looked OK on ADE and the Kindle app, but the .epub file failed ePubCheck 3.0.
Ebook Glue also wasn't able to get all the text from some feeds, but it might work for the feeds you're interested in.