Sorry, I must modify what I said. I USED TO change them all the time.
The tooltips used to be plain-Jane popup tip strings, up through SketchUp 8, and could be dynamically changed.
THEN, with the release of SketchUp 2013, they overhauled the toolbar “look and feel”, and started using an expanded popup tooltip window that uses the tooltip text as a bold title, beneath it an underline, and then the status text as a description.
The tooltip text became static, (and there is a bug in 2013 where mouse-over Ruby buttons do not display status text in the statusbar while other tools are active.)
But I will wish and request that they be made dynamic again, because I have alot of code that changes the tooltip. If they won’t change this, I’ll have to modify alot of code to use the “title/description” protocol instead.
ADD: These changes were not accompanied with updates to the API docs.