It's not a limitation of asp.net, Kentico or anything else; it's simply your design requirement.
I don't understand that statement... If I were doing this in ASP, or PHP, I would make two forms on the site. One in the header that submitted via SSL and one on other parts of pages that need a form, like a cart, or contact form, etc. Even if I was making this via a custom ASP.NET solution, I would make a second (non-server side) form (outside the main server side form in the master page since you can't nest forms) for this login form in the header. This "logon mini form" is not capable of doing this, and my searching finds no other way of doing this in kentico, hence it is a limitation of Kentico right?
Anyway, I don't want to argue that point with you, I want to thank you for the idea of just securing the entire site. The client was happy with that approach. I was concerned about possible performance issues (which is why I normally only secure sections of sites that are in need of "securing") due to the overhead of SSL on the server but this is not that high traffic a site so I am not really concerned.
Thanks,
Matt