How to protect login via SSL but not the rest of the dashboard

It is probably possible, but if you do it the security impact is like not using ssl at all unless all you want to protect is your user and password and don’t care much about protecting against actual hacking into the site.

While user and password are sent only as part of the login form, an equivalent authentication information generated from them is transferred with every request you make to the site in a cookie. Since they are always sent and needed to access the admin there is no much point if you encrypt the authentication information for one page but let them to be sent in free text to other pages.

Unless specifically looking to get your user and password, the hacker that tries to break into your site by monitoring your communication does not care if he got your user and password by intercepting the login info on the login page or by intercepting the cookies on a post edit page.