just gessing here, since i can't realy test now, bust as far as i remember, listItems do not have a scriptUIGraphis object attached, so you can't change their apparence, only that of their parent (listBox).
Yes, you're right, there is no ScriptUIGraphics object for textItems.
Is there not some special character you can add to create an underline
though. I was looking through the documentation, and using "&" makes an
underline under one character (for a shortcut key). Maybe something like
that? Thing is it only appears when the user presses Alt (I'm going to
test that now).