A paper from FOCI 2018 by Arun Dunna, Ciarán O'Brien, and Phillipa Gill on the subject of Tor bridge blocking in China has this interesting suggestion (Section 5.2):
https://www.usenix.org/conference/foci18/presentation/dunna To do this, we write a series specific rules using iptables in order to drop packets from Chinese scanners. ... We use a rule to drop incoming Tor packets with an MSS of 1400. Further investigation would be needed to analyze potential false positives... We note that this method of dropping scan traffic successfully keeps our bridge relays from being blocked and allows our client in China to maintain access to the bridge.
Like https://github.com/NullHypothesis/brdgrd, surely this trick won't work forever, but if you're setting up a new bridge, it's worth a try?
This is completely untested, but I think the iptables rule would look something like this: iptables -A INPUT --protocol tcp --dport [your-bridge-port] -m tcpmss --mss 1400 -j DROP
Then, after a while, check /var/lib/tor/stats/bridge-stats and see if you have any connections from "cn".