Mine is a button so I don't think the cursor properties will work unfortunately- I tried adding it but it just ignores it.
The image for the button is listed in the config xml- I am using one of those, but it's simply an icon for the button and not dynamic (apart from through the 'checked' property)....I might look at changing that property though.
The 'busy' cursor does appear sometimes by itself, but only when the code is being processed, it doesn't appear to show the 'busy' cursor when you are calling a geo-process or when that process is doing its thing....and those are the parts that take the longest time.
So it doesn't look like it's possible but your post was helpful 🙂