If this is going to become a pattern, it probably makes more sense to allow the browser chrome to be manipulated in a way so progress bars aren't redundant (i.e. 'pjax' style loading uses the same bar as if you did a real page load, just like pushState allows for history manipulation.)

It would be cooler to see some guys get together and make a de facto standard, and then implement this type of control into extensions for FF/Chrome.