Today I called the DMV and (eventually) talked with a nice, friendly lady who checked the CARB web site, confirmed the coach is now compliant, and initiated the release of my reg card and tag.
I say 'eventually' because I tried countless times to answer the automated question "what's the License Plate?" We've had the same custom/vanity plate since our first coach in 1995. When I tried responding with ABXYZ, I received the automated reply "got it, continue". There was nothing to continue with, and I heard "please repeat your License Plate", then "please repeat ... and spell out each character" ad infinitum. I finally found the right button to request a callback from a real person.
All the pain and time could be saved if there was a simple way to to input fewer characters than the max (e.g. a question "how many characters?". Or the option to enter the plate via keyboard with "#" or some such at the end.
Meanwhile, since the original registration hold was placed automatically by the "non-compliant" in the CARB database, it should be possible to have it automatically update the DMV system with the change to "compliant".