For Pathfinder, I'm using the ECG slot for AC, the green heart as HP (obviously) and some players are using the lightning bolt for abilities that they can do a certain number of times per day.
Even if we can't add our own images from the art librray, what if it's just a set of basic icons you can choose from a list perhaps, like in the old days of IM avatars? You click on the picture, opens a new window filled with little icons (like a shield, some stars, a clockface) and you click one to choose it for that bar.
