Beta version 6.0 now includes OpenSSH cert but doesn't work when tunneling

Not sure if this is the right spot for beta feedback, but here we go:

I love that version 6.0 now includes OpenSSH cert pubkey used with privatekey. That is great, however when you use Tunneling, it only allows for a separate privatekey for the server you are tunneling through. I do not see support for OpenSSH cert pubkey to use when tunneling.

Indeed, there's no separate box for certificate for the tunnel. But you can still include the certificate into the private key file.
See https://winscp.net/eng/docs/ui_puttygen#certificate

Extra step, but that worked worked brilliantly!