In reply to professorterrible:
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.