Hello,
As far as I know, DNN's security basically works at the tab or module level. You have site-wide role memberships, but the UI for the framework applies these at module and tab levels.
As a developer, you can access all of that permissions info through the API - as you show in your example. But, this is done through custom code and not through the standard DNN interface.
With that said, if you're thinking of making this page, and then loading it in an iFrame, why not just make a standard module with an ascx file. This would let you take advantage of DNN's standard permissions interface.
If you want to have greater control of the design in your module, you can always use a custom skin in the module's edit controls. This is a tip I found in the code for the NB Store module.
I don't think there aren't actually that many cases where you'd want to use a full aspx file in a DNN site. They can be handy for things like a print-view or something along those lines, but beyond that, it's best to stick with the process directed by the framework.
Hope this helps,
Mike