It depends on how the mod items inflict damage. Damage can be inflicted either environmentally (i.e. lava, drowning), by enemies (i.e. zombies, creepers) or by players (i.e. arrows, swords, tnt, splash potions). This is why when you turn the pvp config off splash potion effects don't work on other players, irrespective of other players. It's also why their are separate entities for player fired arrows and skeleton arrows, and why tnt and creepers use different explosions. So it all depends on how the mod is coded.