Thanks for confirming it's not extensible (saves me the night trying to figure it out). I came to the conclusion it's not overridable either (believe me, I tried). :(
I wanted to create a separate section apart from Admin for site users to register/login and do some basic CRUD and periodic updates. I was skeptical about letting them into Admin (and creating a separate permission that would "hide" all other links
in Admin). I was also using a modified Bootstrap template and was finding it difficult to merge the menu scheme therein with the Admin's menu (accordion menu, and the top/parent link needed an href="" to work and since Admin's menu creates a link
it was impossible).
I got over that problem by doing this in my theme's Layout.cshtml:
The Layout has this:
And so I created a Navigation.cshtml with my accordion menu. I then created an empty MenuFilter.cs in my module with:
public class MenuFilter : FilterProvider
I borrowed that from Advanced Menu. The suppression takes care of "hiding" the real admin menu (I'm stuck for now with my own, but it's nothing too complex).
My SecuredArea module has some other code, including a theme selector (again, borrowed from OneStop.CustomAdmin). That module has a Route.cs. Now I am able to have a "separate" (at least it looks that way to the user) themed "secure"/"dashboard"
area that has it's own URL (~/SecuredArea, instead of ~/Admin).
My theme has multiple views (some overriding Orchard's, others just part of the new theme).
Not a programmer, so I'm sure this is not the best way to go..but it's a start. Now just spending time trying to get the shell of this separate "dashboard" set so I can start creating parts to populate it.
I put the [Admin] attribute on my controllers. The other benefit is I can use drivers better in the back-end (my understanding is that drivers are limited for the front-end, requiring .cshtml files to be in the root of ~/Views).
I hope this helps someone.