First I want to start off by saying I agree that Butcher should have the interface to do this if you want to, eventually. But this is different than Crown and thus doesn't need to be a real priority at all.
Crown's case isn't an example of something that literally never matters implemented because it is technically possible - the Crown case absolutely can matter in a game with the +Card token. So it has to be possible because, literally, right now, you can play a game where you have to do that.
Right now, the case of using Butcher instead of the Buy phase as your mechanism for spending all the Coin tokens literally never matters - it's currently not possible to play a turn without any Buy phase whatsoever (even if you can't buy cards), so you will always have an opportunity in a Possessed turn to do what you are suggesting. So it isn't a thing that immediately needs to be fixed.
Crown still should be fixed by the way with that "action phase / buy phase" button or warning - we just clearly need the Action phase thing to be possible, because it is actually possible for it to matter in games playable right now, unlike this Butcher thing.