Its been clarified many a time over that you cant use things like BCS to get an extra challenge off in the epic phase, since there is no mechanic arrow to cycle around in the flowchart. If you want to read it there is a giant thread on it, its why War of Five Kings no longer says what it has printed.
Fully aware of this, what I am wondering is if there is a player action window before you declare your attackers and start a challenge? To play things like Nightmare, or say... use my Shadow Politics to get an intrigue in before the actual epic phase framework challenge as outlined by the epic event takes place. Its slightly different than the BCS situation because its not granting you another challenge (ie, one in addition to) but actually giving you the framework to make an intrigue challenge, or another player to do it, as a player action that normally wouldn't be capable of doing thing. Its usable in any phase (except not in the middle of challenge already happening) so if there is a player action window there, can I use Shadow Politics to get off another intrigue challenge in this phase?
I expect a no, but a man can dream. I think it would be a nice way to catch my casual players off guard, for the luls.