If you dig through the main console form using Reflector, you can see that the health TabButton is a special case. The WHS Console keeps track of it separately, and overrides the Paint handler to draw the correct icon based on the current health state.
The TabButton is what WHS creates when it instantiates your Add-In; we can't reference the TabButton that WHS creates from our add-in (the list of TabButtons is private, for a start). Because of this, we can't override the Paint handler to draw our own images.
I think you're out of luck on this one.
Tentacle Blog:
http://www.tentaclesoftware.com/blog/
WHS Disk Management:
http://www.tentaclesoftware.com/WHSDiskManagement/