I've just completed, and paid for my O-A. Screen says 'Please stay on this page. It may take a few minutes'. It's been like that for about 15-20 mins now. How long did you have to wait for the receipt?
Final comment. I've just answered my own query. Where the layover is more than 6 hours, you are considered a domestic passenger and depart the country in BKK. Therefore, my original question does not arise. I have booked a hotel in BKK. Re-entry permit is not affected, although I understand it will cost 200 baht more than in Chiang Mai.
An easy mistake to make, as the form ostensibly requires a 'visa number', not a re-entry stamp number. I only mention it to help others not make the same error. No big deal...