"I'd be interested to know if you could cram 30.2mm (ISO) cups in place of the 30.0mm (JIS) originals, I didn't try that"

Sheldon Brown seemed to think so. See headsets on the Harris Cyclery site about JIS:

JIS One-Inch Threaded Headsets for Bicycles from Harris Cyclery

Never tried it though. Would be interested to hear from someone who has.