I've discovered that if you use an external tool to generate a Certificate Signing Request (.csr) such as DigiCert CSR Tool (or do it manually with openssl), you can customize the relevant fields and then pass it as a sort of configuration file to mkcert with $ mkcert -csr <domain>.

I have not tested using a CSR for initial Root CA installation, but $ mkcert --help implies that it should be possible.