[XP] How to change an item's icon?

I'm putting together a custom menu and I want an item that is equipped to change to a different icon.

Is there a way to change what icon image an item is associated with within a script?

Yes. You could change the icon in whatever method you use to draw it.

If the window only ever draws items with a modified icon, you may need to only change the draw_item_name method. If the window draws some items with the normal icon as well, then you'll need to modify the method which draws the modified icons directly.