I'm not sure you'd even need a custom redirect module. I think you could just use security at the feature instance level on your download form page to do this. As Joe said it would have the login control/registration/explanation on it (these modules flagged as visible only to anonymous users), and your download form (module visible only to authenticated users). After they register/sign in, and redirect or navigate back to the download form page, they will just see the form on the page.
This all presupposes that the download form page is a mojoPortal page, not a custom aspx page. If that's the case, you could use an intermediate page to do the same thing, with a "go to download form" button or link exposed to authenticated users.
Jamie