Hi!
I just started a configurable module for modalframe that came from the need to have the login links (no matter where they are on the site) be modal windows. As I started writing it I wanted to make it configurable and it started to turn into a bit of an API itself...
In any case, I wanted to make you aware of it and maybe get your thought on including it here. It kind of duplicates the modalframe_register as it handles that link as well, but it's configurable instead of being hardcoded.
I also (for kicks) added a drupal_alter() to the function that defines the list of links => form_ids that are originally defined. Not sure if this is even a good idea, but seemed fun.
Anyway, have a look at it and let me know what you think!