Bukkit

I use both MythicMobs and DynamicDifficulty on my server. They work well together, although MythicMobs has a level system that would work perfectly with DynamicDifficulty. You could create mos that gain custom abilities as the difficulty increases, which would open up a lot of possibilities.
I don't code plugins, but it should be not too difficult to implement since there's an API.

How it should work:
An options is added to the default player config file at the top, while each custom mob is shown below the vanilla ones.

scaleMMMobs: (boolean, defaults false)
#Vanilla
blaze:
  setting: auto
...
#MythicMobs
CustomMob1:
  setting: auto
  manualPerformanceLevel: (MM level)
  autoPerformanceLevel: (MM level)
  maxIncrement: 1
  minLevel: (MM level, defaults to 0)
  maxLevel: (MM level)
  twicePower: (MM level)
CustomMob2:
  setting: auto
...

The minimum and maximum levels are used to set the boundaries of how far the scaling will take effect.
twicePower tells the plugin where 200% power is, which is probably needed to scale properly.

Link to the plugin here:
https://www.spigotmc.org/resources/mythicmobs.5702/

I made a suggestion on their forum also.
http://mythicmobs.net/forum/viewtopic.php?f=12&t=349