Allow the login page to run under HTTP?
This will obviously be introducing a big security risk to a page where users enter their login credentials, which is generally a pretty bad idea. I would suggest no, but it depends what information the site will provide once logged in. If there's no accounting, no personal data, etc. then maybe it's okay, but i still wouldn't do it.
Edit: Updated due to question change
As stated, a static landing page with a link to the secure login page is a good solution here. It would be advisable to keep the http-to-https redirect on the login page though, as some users may try to type the address manually from memory, be using anold bookmark, or using a cached link from a search engine. Alternatively, the http login page could redirect to a different landing page, alerting the user about the insecure page they have navigated to, and also serving the https login link.