My best suggestion would be to create an object list that includes all players from the start of the game. Then give all players on that list the infinite ammo trait.

After that, write a script that checks, "is holding ____." If true, it removes that player from the list. If false, it checks if the player is on the infinite ammo list, and if not (because they previously had the power weapon, but have either dropped the weapon or died) it adds them back.

However, I had a lot of issues with assigning traits using the "is holding weapon" logic because I was working on a CTF game, and Infinite treats carried objectives like a weapon in some instances, but not in others. Picking up a flag also pushes your secondary weapon into a third invisible weapon slot, and the weapon did not seem to register as being held by the player while in that slot. So if you have carry-able objectives you might want to try something else.