With the August release Ex Libris introduced an improved interface to letter customization (and "component customization"), where the labels and template for a letter are accessed in tabs on the same page. According to the release notes the old pages will be replaced in the near future.
While the new pages probably are better from a user perspective, they also seem harder to script as the complete list of letters is no longer accessible from a single page, and at least I can't find information on when a template was last updated.
Do you plan to continue development of slipsomat to work with the new pages?