The overhead of SSL is minimal and not something you're going to notice on modern hardware. As a result it really does not make much sense to have an SSL listener just for the logon page.
While it probably wouldn't be that difficult to modify the Managed Reporting logon page so that it always submitted your logon to SSL, you would then need to redirect the request back to non-SSL after a successful logon.
You'd have to do the same for the Reporting Server logon.
This is a lot of headache and customizations for no real benefit and it could introduce additional unforseen issues into your environment.
If you need SSL for your logon page, you should just add it for the entire site. I'd be extremely surprised if you had any noticeable response time differences between SSL and non-SSL with any computer system manufactured in the last 10 years. Even with older systems the speed difference would be minimal.
WF 71.x, 76.x, 7701, 8.0 Beta OS: Linux, Win2k3, Win2k, Win2k8, WinXP