Here I am! I've been to China 5 times for business (among those 3 times to HK and 1 to Macau).
I have to say that in Beijing Airport you can pay at duty free with Euro and you can have euro as change.
At shops it depends if they have change back or not.
I use to have the same trick everytime: Ask to clerk at Duty free or exchange offices if they need fivers or tenners in exchange or viceversa.
Sometimes they stare at you suspiciously. Most of the time they say no, but in few cases they accept.
I did so at Zhuhai Ferry terminal without any problem.
Same at HK Ferry terminal, once they accepted and twice not, to exchange a

into smaller note.
In Macau airport (MFM) no chance to have any euro exchanged unless I changed back MOP into EUR.
At HKIA (HKG) same situation like Beijing.
In short. Downtown it has been almost impossible to get euro (at Silk Market in Beijing they accepted euro and gave back CNY as change!!), at main airport you should get something!
Enjoy your stay!!!
PS: I've been to WAW airport as well, I was able to exchange

into smaller pieces at the old terminal facilities exchanging at "exchange money". As I suppose you won't exit from departure hall, at duty free they accept euro (so same trick to exchanged) and give euro as changeback.
I can't remembar the matter about the brand new notes... but for sure brand new ones are better then used ones to the eyes of people not very used to EUR (this last for every foreign people!).