Could change all potencies on abilities above the synced level of the dungeon/trial/raid to 1. Seems like that shouldn't be hard to program since it wouldn't actually be trying to scale anything. Either the dungeon/trial is at or above the ability level so you get normal potency, or it's not and you get 1 potency. It's not like players who want to use their end game rotations are going to care about the potencies, they just want to be able to use those abilities. Abilities not at full potency could be greyed out or given a red outline to alert those players who care about potency to avoid them (probably most relevant to healing).

Yes, I can imagine the chaos that would create. It also has the potential to make those low level dungeons feel more difficult thanks to many wasted GCDs, which isn't necessarily a bad thing for the new player experience. A game can rapidly get boring when too much of it feels faceroll and its going to be a lot less faceroll if part of the party is getting in a 200-300 potency attack in every third GCD instead of every GCD.