It's not useless at all. He asked (and you didn't answer)
WHY do you feel you need to do this?
What actually useful purpose would this serve?
It's entirely outside of the scope of what valet aims to accomplish, so expecting valet to support it seems somewhat foolish to me, really...
Now.. that said. You could drop an nginx server config file into
~/.valet/Nginx configure the entry in your hosts file
/etc/hosts and restart nginx to force it to work. But without rewriting valet's code you're unlikely to get it to work in any sort of automated manner, nor should it really as -- like @Snapey alluded to, there's no real value there -- the site should be agnostic of what domain it lives at, aside maybe from something in the site's config or .env files.