Here's my first shot at your "alternate value sent to server" todo item. With this change, if a date picker element has a parent form, we bind an event to that form to reformat the date field's value to the format defined in SETTINGS.format_submit before submitting the form. You can also disable the behavior by setting format_submit to false.

I'm still tweaking it a bit, because there are a couple of minor issues with this approach:

• You can see the field value change for a moment before the new page loads. Also, when you navigate back to that page, the value remains reformatted (though that may or may not be a problem).
• I haven't tested it much with AJAXy forms. I want to make sure it doesn't cause problems with other submit event handlers.
• It might throw an error on non-<input> elements when it tries to find a parent form. I still need to play around with it, but it looks like I can solve this by using P.$element.closest('form') instead of P._element.form: http://stackoverflow.com/a/991371/349353

EDIT: Looks like you're already ignoring elements other than input, so that last point is moot. That's good, because P._element.form is quite a bit faster: http://jsperf.com/finding-an-input-s-form