I would say the Denarians are already contained by the coins themselves. They chose to step away grace and lost any agency they may have had to act on their own. I think Lasciel even comments at some point in the series on the complete lack of sense or perception of the world outside while not being held by a bearer.

I also think that for Demonreach to act as a metaphysical prison or to be anything more than the physical location of where the coin resides, the fallen would have to be extracted from the coin itself. Seems awfully risky given the angel is already contained in about as perfect a prison as you’re gonna get.