OK, I'm now following Bitnami MU Domain Mapping Instructions.
Site is added as per instructions. Site still shows ERR_CONNECTION_TIMED_OUT. I guess I'll wait 1 hour for propagation.
Edit: Cannot add another reply.
OK, playing with the DNS has opened up some pipes.
The new domain works, but www. prefix redirects to non-www automatically.
I need both to work. I wonder if it's within the plugin or in Bitnami.