Y'know, maybe I'm going about this the wrong way.
If anyone knows what the coating is on the firebox and lid, that would at least offer a clue as to what might work, or what to avoid.  For example, if it's an porcelain/ceramic coating such as used on the flavorizer bars, I don't see why I wouldn't be able to use something like EZoff oven cleaner.  If it's an enamel coating (paint) then I'd need to be more cautious.
Short of any other feedback, I guess I'll hit up the Weber folks and ask if they have any input.