I’ve just released a new API for Bukkit which allows plugins to assign various metadata values to mobs to be used by other plugins (or the same plugins in later events).
Spawns and Events will be updating soon to use this API instead of local code. Spawns will store the time and location of the mob when it spawns, along with the hub/pack/etc. ids. Events (or any other plugin) can then read and modify this data to further customize the mobs.
The good news is this will separate the plugins and allow many more possibilities.
The bad news is this means all of Spawns‘ custom mobs will be lost and only the ones spawned with the new Spawns while Mobsistence is running will be saved. If this would be incredibly damaging for your server let me know and I’ll see if I can knock up something to convert the formats.
You can find its BukkitDev page here.