I never played with a Stag lower so I really don't know...
The photo below shows my Bushmaster on the right with a 9mm hammer installed:
Notice how the shelf around the selector is below the height of the selector.
If your lower has extra material so that the shelf is higher then the selector you will likely have some problems.
Good luck and let us know what you lean as I will update the LL page if the stag lowers will work.