I had a look around the react-native docs and found this. Specifically it says
The - (dispatch_queue_t)methodQueue method allows the native module to specify which queue its methods should be run on
So I added one to my RN module here, and I can see at least form the logging that it looks like all method are now invoked on my new thread (called RNCBL.Queue), for example:
[info][tid:RNCBL.Queue][ReactCBLite.m:38] Launching Couchbase Lite...
It works if I specify the main thread. Thanks for your help.