Haha! I can't believe this worked.
If you use Ibis Reader, you will have seen the "Get Books" link. This allows you to view OPDS catalogs (lists of web-accessible ebooks). The Feedbooks catalogs are pre-installed, and some of you may have set up a Calibre/Dropbox OPDS catalog of your own library.
On a whim, I used the "Add Your Own Catalog" link to add the WebScription Stanza link
It worked! I can access the Baen Free Library directly from Ibis Reader! In "Get Books" I click "Baen" (or whatever you named the OPDS link), then "Baen Free Library", then the "Read" button next to the book I want to add to my Ibis Reader library. Yay!
I don't think it's possible to log in to the Webscriptions "My Books" area and download any Webscriptions books you may have bought, but I think just accessing the Free Library catalog is a pretty neat trick.
Edit: When I use the URL
I can bypass the general Webscriptions splash page with the defunct "My Books" link and go straight to the Free Library. This is a bit hack-y in nature, though, so don't blame me if your computer goes boom