Yes, it's been needed for a while, and since I needed it pretty badly in my hack, I decided to take a look at it and found it pretty fast. Note that these room transitions are for
palette changes only, not, for example, going into the Maku Tree map. Here's what the editor window looks like.

It's already done. The map shown there is the first graveyard map you encounter. All you have to do is set a map and the direction you enter the map to set the transition and it's all good. The old palette fades into the new one properly.
~Lin