Hi,
a reply just from my experience.
I have done the icon thing in two different ways:
1) Referring in CSS the ids given automatically to the toolbox items (':1', ':2', ..). The problem is that is hard to maintain: every time you change the toolbox you have to change the style file. And if you change the toolbox at runtime, it is still harder.
2) Loading the icons as a font (that seems to be the cool thing to do nowadays). So I just changed the names of categories to include the icon as a character.
You can see how the result here
By the way, I agree with you that it makes sense to have a method to configure the id/class given to the visual items.
Miguel