Well, obviously a static webpage is going to have no way of knowing
whether there was any authentication done in the first place, and a
model, of security through obscurity by only making that static link
available on a GAE website that itself requires authentication isn't
really security at all.
If you really want to make the user visit GAE to authenticate every
time instead of saving the redirected link, using expiring session IDs
is fairly trivial, even if it is complicated a bit by having to pass
session data between 2 different systems.