asp.net custom security policy

I have been playing with predefined full trust, high trust and medium trust. And I am trying to make a custom policy set.
Right now what I have been able achieve is to define a permissionset and define classes in order to grant permission, any classes undefined are restricted by default.

I would like to define a security policy such that if any namespace and assemblied that is not explicitly defined, it's permitted to run. Is this possible.

...