@barbarouspony On another thread this was asked and I suggested an idea that I believe worked for the person that was asking. Since Valet relies on directories to resolve the domains, I suggested that the person create a symbolic link to the directory with their installation, and that they name this linked directory the subdomain they wanted. So like if you application is in "domain" directory make a symlink and name it "subdomain.domain" with something like:
ln -s domain subdomain.domain
That will make valet find your subdomain. Supposedly this worked for the user that needed it. Now, since the latests versions of Valet use Caddy server, maybe they will be able to find a way to make this simpler? I think it provides more flexibility. But for now this seems to work.