In other words, if you want to be discovered on Mastodon using your own domain, you can do so by copying the contents of https://<your mastodon server>/.well-known/webfinger?resource=acct:<your account>@<your mastodon server> to https://<your domain>/.well-known/webfinger.

One caveat: this approach works much like a catch-all e-mail address. @anything@yourdomain.com will match, unless you add a bit more scripting to only show a result for resources you want to be discoverable.

Edit: Seems there is a GitHub issue which requests custom domains as well.
Edit (15 Nov 2022): Folks have been using the approach of serving up webfinger on a different domain through proxy setups, e.g. using CloudFlare.