That will never happen. SSL is based on trust, and the trust root will never blindly delegate to whatever happens in random LANs. Subdomain is 100% the right approach for internal network.
It can and has already happened. You can make your own root ca. Internal domains need internal root cas. Is it a pia to setup yes. Do I have it installed on my unrooted android phone and linux computers? Yes.
Edit: I didn't see the dhcp part. But you can still make your own root ca
The maintainers of DHCP can't even be bothered standardising a query to check if an address is currently in use, doubt they could take on being a CA at the same time