After doing some reading I am answering my own question.
There is no way to get rid off the cert error. It will be like hijacking ssl cert.
The problem I have is a industry wide problem of captive portal mechanism. It was good until it lasted…or until more and more companies started using https.
the best bet is to allow 80 only for captive portal redirection. That’s the best can be done to avoid cert warning.
Hope this helps someone like me under frustration.