This is a little feature that is missing and seems rather obvious. How about a small drop-down menu with some times? I think, simply for lack of convenience, most people almost never change their online status, and so we end up inviting a whole lot of people who aren't actually paying attention.
I think the most useful implementation would be an automatic status change after a certain time of the PVP.net window being out of focus. This would account for people who are at their computer, but actively engaged in something else, which is probably most people.