Those are the only things that it can be used on. In fact, those are the only graphic elements supported by the GBA. Everything else (blending, windowing, rotation, etc.) is just an effect of some sort.
No, that would be way too hard to manage. I utilized a feature called alpha blending. The background of the textbox is blended with the map but the text doesn't blend. It took me a long time to figure out how to do that. :D
I personally like the coloured textboxes more,but I love how it's never been done before.
Did you insert a routine that changes the PAL of the stuff under it?