Add a "loading"-class to computed elements that can update via AJAX

The twig computed element is just fantastic and the ability to ajax-load it is even more fantastic.

I was able to build an order webform with inline price calculations via AJAX: terrific!

But, since calculating all the prices takes about a second, it would be real cool to have a class on the element that is reloading, or perhaps on the entire form. That way we can do some theming, adding a loader icon for example.

Would that be possible?