Just to update- as moondog as correctly said- it all worked out. Certainly having a printout with me of the regulations made the difference- I was on the very early UL flight this morning from BKK to PVG- and the UL people had never heard of such a thing. In fact they showed me a "book" they had of email printouts where someone had been denied entry because they were more than 24 hours. In the end they took a photocopy of the page from the China embassy and said "OK" but very reluctantly. Without the printout I would still be in BKK. Once arriving at Shanghai it was all very easy and straightforward.

I think the only problem with this is that at outstations very few people know about it- so definitely do check in very early for your flight (it took me 90 minutes to sort this out at 4:30 this morning)- have all the documentation printed- and be patient and ready to explain.

I can see that if the wrong supervisor was on duty it would be pretty easy to get denied boarding.