I really don't have an answer for your question. But Marketo has to install the certificate for you after you purchase it. You have to email firstname.lastname@example.org to request the install.. I'd ask the same question in that email too. I'd be interested in knowing their answer as well.
This is important because if you do have traffic going to Marketo LPs on plain http:// you would want to make sure the cookies are valid back and forth (and they are).