If you need a certificate to prove you own a domain, changing DNS TXT records for that domain, or serving a secret, from that domain, proves you own the domain.
If I need a certificate that proves I am the corporate entity on some signature, say, "PhotoStructure, Inc.", there isn't some magick TXT record I can add that uniquely identifies me as the owner of that business.
If you need a certificate to prove you own a domain, changing DNS TXT records for that domain, or serving a secret, from that domain, proves you own the domain.
If I need a certificate that proves I am the corporate entity on some signature, say, "PhotoStructure, Inc.", there isn't some magick TXT record I can add that uniquely identifies me as the owner of that business.