Basically, an additional set of policies for realms to enact, with the usual cost. Since the number of combat stats and skills is fairly limited, I think it'd be best if the buffs applied to combat moves so there's some difference between realm. The idea is, you get effective points in a move even over 30 total, but never over 5 per move, as long as you have at least one copy of the move in your deck.
Unlike civilian buffs, these combat buffs should only apply to full realm members. The point is to give the realm a home field advantage in PvP in small skirmishes. To even the field in a proper battle, I think it should be possible for the attacker to bring their realm's combat buffs with them at a cost. I'm thinking new realm "magic" called Campaign or War Party or something, which only the king can use, costs authority to activate and has an authority upkeep while active. The effect is that the king gets an aura (like 50 tile radius) that grants his realm members their realm combat buffs even outside their realm.