If you could, like MCEdit, open a .dat file using NBTExplorer.
Hmm... I'd probably sooner just build it right in, because MultiMC contains an NBT library. Adding some UI for it wouldn't be that hard...
NBTExplorer is here: https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/minecraft-tools/1262665-nbtexplorer-nbt-editor-for-windows-and-mac
Thank you.
Most helpful comment
Hmm... I'd probably sooner just build it right in, because MultiMC contains an NBT library. Adding some UI for it wouldn't be that hard...