I'm putting this in as a "support request", but if this isn't supported, please change to "feature request".
Essentially, some form submissions should really have some sort of confirmation message. For registration in particular, this lack of confirmation message is a pretty bad UI design flaw, since a less savvy user might not understand what happened after form submission.
There are reasons why I'm sure this would be complicated to design an admin UI around, but we should at least have the ability to attach a callback to the form that modal_forms would look for, which would target a confirmation message and "Close" button (by default). Does such a function/technique already exist?