Added WebFinger support to my email address using one rewrite rule and one static file.

gistfile1.txt

[aaron@parecki.com www]$ cat .htaccess 
RewriteEngine on
RewriteCond %{QUERY_STRING} resource=acct:(.+)
RewriteRule ^\.well-known/webfinger /profile/%1? [L]

[aaron@parecki.com www]$ cat profile/aaron@parecki.com
{
  "subject": "acct:aaron@parecki.com",
  "links": [
    {
      "rel": "http://webfinger.net/rel/avatar",
      "href": "http://aaronparecki.com/images/aaronpk.png"
    },
    {
      "rel": "http://webfinger.net/rel/profile-page",
      "href": "http://aaronparecki.com/"
    },
    {
      "rel": "me",
      "href": "http://aaronparecki.com/"
    }
  ]
}

Alas according to the documentation RewriteMap cannot be declared in a per-directory context including .htaccess. So one probably need to name their json files with %40 instead of @ or symlink them because some clients (e.g. Mastodon) send unescaped requests. This also requires to support both : and %3a

RewriteCond %{QUERY_STRING} resource=acct(:|%3[Aa])([^&]+)
RewriteRule ^\.well-known/webfinger /webfinger/%2? [NE,T=application/jrd+json;charset=UTF-8]