Maybe you can cook something with this :
Len(string)
Returns the number of characters in the string.
Check the number of characters in the tooltip text and then adjust the text object's size and frame size?
You'd probably have to know the width and height of the police you're using though.
aaah i don't know i'm just a noob ^^:
Edit : Or maybe you can try to do it the html/css way : make small images for the corners (or just one since you can rotate it), and one for the frame itself that would be stretched or repeated until it fits the size of the tooltip. Because if you use 1 image only it would be blurred. And then add a Tiled Background.