I hope suggesting this isn't stepping out of line. But I was thinking, would it be easier to freeze a dimension by providing your own world class that overrides the update methods so that they don't run (or run limitedly) when the world doesn't have power?

As the implementation would appear to be not supported that well with the new Dimension system in forge / mc as of 1.14 (partially in 1.12) this would have to be brought up again referring to a newer version of the game.

Yes, but how can one provide their own World class without ASM?

Forge's DimensonManager class seems to have a setWorld method. And this method is called in the WorldServer constructor, so I think you should just be able to extend the WorldServer class and provide a world that way.