This thread might do what you want with a patch that started out as a request for the time to be displayed, but ended up giving us a very nice way to add the time, date and battery level on every page. I got used to the time display from my Sony 350 and always missed it.
I only display the time and battery level, but you can arrange it however you like, even dropping time and date and only displaying the battery level if you prefer. Mine looks like this: