Or simply have action: MOBSPAWN affect mobs spawned by OtherDrops.

Is this supposed to work or does the mob have to actually spawn for spawnedby: to recognize it?

Following entries are for gluton (hangs out in IRC). He wants to spawn groups of zombies (works), but some of the spawns to have equipment (works only on vanilla zombie spawns)

When calling action: MOBSPAWN it only works on mobs spawned normally by the server

    ZOMBIE:
      - action: MOBSPAWN
        drop: ["zombie@eq:hands:STONE_SWORD!!eq:chest:LEATHER_CHESTPLATE", "ZOMBIE@60h!!eq:head:LEATHER_HELMET/50%"]
      - action: MOBSPAWN
        drop: {"ZOMBIE/2/30%", "ZOMBIE/3/15%", "ZOMBIE/4/5%", "ZOMBIE/5/1%"}

The intended setup could work by doing complicated drop groups with different equipment setups, I just think it would be much simpler to catch all mob spawns and say every zombie has x% to have this equipment.

The problem here is that if mobspawn works on OtherDrops drops it'll form an infinite loop and keep replacing the zombies with 2 zombies or more - before you know it you've got thousands of zombies on the spot and the server crashes.

This will become possible with custom mobs (when they are eventually added), rough example only:

custommob:
  zombie_stone&leather:
    - ...desc, config, etc...
  zombie_60h_leather:
    - ...desc, config, etc...

alias:
  zombie_list: [zombie_stone&leather, zombie_60h_leather/50%]

otherdrops:
  ZOMBIE:
  - action: MOBSPAWN
    drop: {"zombie_list/2/30%", "zombie_list/3/15%", "zombie_list/4/5%", "zombie_list/5/1%"}