I'm with the Hoosier. I've banked coals on almost every cook in a 2011 green otg, many of them extremely high heat, and I've never seen the slightest sign of damage or wear. I'm not saying it doesn't happen, but either it takes a very long time or there's something different about the newer kettles.
It seems the rust develops because oxygen and moisture are able to get through the enamel and cause corrosion. If high heat causes expansion & contraction at different rates in the metal and porcelain then I suppose cracks and chips could develop. I just haven't seen them.
I have an '88 red kettle, and I avoid banking coals in it because of the concerns discussed on this forum. I want to preserve it as best I can. But it's likely that I'll always bank coals in my main cooker. It's just part of how I achieve high heat while maintaining two zones.