Just pass in the name of a json file as a CLI input (or default the name and act on it if present or use it if indicated [e.g. /U == use json.config]).
This doesn’t answer your exact question and I haven’t done this with webfingers, but I’ve done this with a reverse proxy like nginx (or traefik) and no special DNS tricks. Your example.com will point to 1.2.3.4 IP and then the subdomain routing is handled by the reverse proxy. I’ve had upwards of 8 different domains and subdomains all running on a single box taking advantage of docker containers.
Hmm. Ok, but mastodon and pixelfed are unrelated services at the authentication level. When you hit the home page of each it’ll ask you to authenticate. Even if you use the precise same info (e.g. name, email, password even), each one will be authenticating separately. Or am I missing something still?