I had some limited success with preloading a relays.dat - RFE: Client installer reference relay.dat at installation time
I had to install the client (which fails to find a relay), stop the client, drop a relays.dat, and then restart the client.
Using the clientsettings.cfg to prefill the relay configuration is ideal, but can get you in a situation where you either have a large number of distinct clientsettings.cfg files for each environment, or use a script to generate the clientsettings.cfg at install time. I’m using a script with ‘wget’, or you could use ‘curl’, to iterate through a relays list, try to connect to each, and generate a clientsettings.cfg based on which responds.
I tried using the TertiaryRelayList setting, but there seems to be a limit of 256 characters for a client setting so I couldn’t just put every relay in the list.
The thread I linked to was an RFE for bundling a relays.dat with the installer, vote for it if you think it’s useful for you!