Custom Monsters despawning in Peaceful mode

What I expected: There would be a way through NBT data-e.g. NoAI:1-to allow a custom monster to persist in a world set to Peaceful.

What happened: I was unable to find any setting or combination of settings which would allow the monster to persist for more than a single tick.

Discussion: Monsters with NoAI:true, especially, are not actually hostile. Invulnerable:true or PersistenceRequired:true might be other tags for which a case could be made that the monster shouldn't automatically/immediately despawn. Since only custom monsters would be affected, there would be no perceivable change in vanilla worlds. Mapmakers who previously used setting to Peaceful as a shortcut to remove custom monsters could instead /kill them, or tp them into the void, etc. Having the option would make certain visual effects-a Spider NPC in a peaceful adventure, for example-possible which are currently difficult or impossible.