The power consumption issue isn’t with the card itself as @anon81945988 alluded to. It is with the controllers and retimers not going into a suspend state when no display is connected. Or at least, that is what I have understood from reading this thread.
This isn’t to say that your idea wouldn’t work. It would. Just not for the reason you think. If you can disconnect the expansion card from the TB4 controller, then it might as well be unplugged/removed. So things will move into a suspend state as they should.