Changing Pro Sites Access from "at least this level" to "only this level"?

I understand that Pro Site levels work by allowing access to any item AT LEAST equal to the users's level.

Has anyone customized it to work as mutually exclusive levels so that only items I set in Level 3 are accessible in Level 3?

Yes, there will be some duplication when I need to add the same theme/plugin to many levels, but that would allow me to sell completely different types of sites from one installation. One specific plugin may only be germane for Level 3, while another specific plugin might only be needed for Level 4.