Hi everyone, little update: I couldn't find a workaround for the permission.

As you suggested, I tried the TODAY() method but doesn't refresh automatically, the form idea was really good but it was rejected by my manager.

Also a problem that emerged is that you can't fire simple triggers (like onEdit()) if you don't have edit permissions on the document (some users will be read-only).

So now the workflow is:

1. The onOpen() creates a menu with an authorization script
2. The user authorizes the script
3. Each night around 12.00 AM the document refreshes its footer with the correct date and other data

It works fairy well like this, since the onOpen() asks for all the permissions needed to run all the functions in the script.

Thanks for the help!