I don't think this is possible. For OLE DB, I seem to recall that there are registry settings in the other direction, to disallow Optional. Which makes sense, this is an insecure option.
A better option would be to install proper certificates and distribute these. Or install a certificate with a trusted root certificate.
In any case, one the user has set an option for a server, SSMS seems to remember it, so it is a passing phase.