In case anyone else is looking for this kind of functionality, I managed to find a way.
When the Ammo Unit is created, the Birth Effect in the AM Ability applies a set that gives both the Owner and the Ammo Unit a unit tracker and each stores the other in the tracker. This way, it is possible to refer to an Ammo Unit's Owner through its Unit Tracker behaviour, with effects like Enumerate Tracked Units.
You can adress a magazine unit with the Iterate Magazine effect. You can return a unit in the magazine with the Load Container effect. Just take note that there is a bug that prevents unit from being Released from Magazine effect if they were put in the Magazine from a Load Container effect. It's a bug that won't be solved until I can reach out to a single employee, when Blizzard Activision will settle their internal mess.
Specifically, what I was trying to do is refer to the Magazine Owner itself, via an Ammunition Unit I'm not sure how to go about that. My understanding of the Iterate Magazine effect is that the effect has to stem from the Magazine Owner and will then effect the Ammunition Units. What I was looking for is a way to do the opposite, to have an ability target one of the ammunition units and for the effect tree to jump back to the owner.
It is only possible if the Magazine unit is internal, in which case "Outer unit" is the one location to look for. Otherwise, the only method for an external magazine to track its owner would be to have a behavior on it created by the magazine owner, and when it's time to search the caster, you look for the one who gave the behavior as caster required (Unit Compare Behavior Count Validator). Otherwise, there is no method (or maybe unit tracker) that can directly trace it. It is however one of my wishable content request to the dev team.