The problem with this is that you can't present the original form with the values filled in if the form contains file upload widgets or password fields. Browsers often refuse to fill those out because you can be tricked into uploading files you didn't intend to, /etc/passwd or something.

But storing all the posted content and then forwarding to the processing page for the original form may work somewhat. Usually when you hit a protected page and you're not logged in, you get forwarded to the login page (via an internal or external redirect). You will have to ensure it's an internal redirect so you still have access to the POST content or don't redirect at all, intercept the request and present the login form at the original URL they requested.