@reiver@reiver ⊼ (Charles) 
Out of curiosity ... could I use this system _today_ to make my handle say @felicitas@pojtinger.com as a Mastodon user? Assuming I have control over the web server running at https://felicitas.pojtinger.com/?
You would have to control the web-server running at pojtinger.com
When the Fediverse sees the Fediverse ID:
＠felicitas＠pojtinger·com
It will try to do a WebFinger look up on pojtinger.com
That WebFinger service could redirect the request to felicitas.pojtinger.com or another other place.
And then you could have Mastodon running on another domain
If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.social/users/reiver/statuses/116118147582372270 on your instance and quote it. (Note that quoting is not supported in Mastodon.)