On Cyrodiil is applied the martial law, guards would simply cut your hands making you unable to use weapons, then you would be sent to the front as meat shield.

But for jokes, I think they avoided it thinking to the second part of Justice System, that introduces PvP within the same Alliance, and because the main focus should be AvA.

To have something related to Justice system in Cyrodiil, it would be interesting a sort of pillaging - plundering mechanic when you conquer keeps, resources or when you approach a village.

Another strange idea could be have an huge underground Outlaw Refuge where outlaw players and guard players don't stick to their Alliance so outlaws are from AD, DC and EP, same for guards (so just a two faction system).
This could be an issue for the war above, so imho it would be better to create something similar as a battleground/scenario not related to Cyrodiil and with usual mechanics of this kind of instanced battles.