Okay, it appears my last blog post provoked a large reaction. I think the conclusion was that the libnotify notification was indeed crack-tastic, and that we should probably add this information to the tooltip, or just try and 'Do The Right Thing'.
For the moment, I've added a small “Battery charge profile is estimated” to the tooltip, but I'll try to fallback to the rate calculation time remaining, although that might mean dealing with all the duff data that comes with that. Cheer for all the feedback.