Okay, I've finally done it. If you log into user services now, there should be an option to use external email, and a text box where you can enter the list of mail servers to use. Don't enter priorities, just the five google mailservers in order. In other words, put exactly the following into the text box.
As this is new, it's not completely tested of course, so please let me know if you have any problems or see any errors.