Setting HTTP header variable from ASP page?

Does anyone know if it is possible to set/create a custom HTTP header variable from within an ASP page?

As an example, I have an authentication page that validates an end-user's id/password against a directory. Once that has completed successfully, I need to create or update an HTTP variable that can then be passed back to the server, where is is read by additional application code. The HTTP variable could be an existing one, like remote_user or a custom variable that is created.

The limitation is that the "backend" application code that gets executed can only read HTTP header variables, and not cookies/etc.

Any assistance is GREATLY appreciated.
- K