Depending on how many sites you have, you could use the relay fail-over setting. This client setting allows you to set a string of relay addresses, and the client will try them all in order until one works.
So a single setting in your clientsettings file that basically lists your site relays.
Once connected you can revert back to auto select, or a sub-version of auto select called relay affiliation, or have policy actions enforcing manual relay settings.
Again, you could use GPO to shove this in the registry for those that have already deployed and are in ‘no-man’s-land’