Forms Membership Timeout - Losing Post Data

I have some web pages which are a bit heavy, and users can sometimes post the page back after the Forms Membership timeout has elapsed.
When this happens, they are taken to the logon page, and then redirected back to the posted page on authentication. But all the posted data is lost, and they have to begin the page entry from scratch.
After spending some time googling, it seems the only way round this is to capture every incoming request in Application_BeginRequest (or some other Global.axax event), and check if this is an incoming timed-out request.
If it is, then the posted form data has to be saved to session, and the page redirected to a custom login, where after authentication the page has to be redirected back to the original form with data filled in from session.
I am just stating to implement this.
1) Is the above the best method ?
2) Is the Application_BeginRequest the event to go for to test every incoming request ?
3) How can I tes ...