It is usually super-lame when clever tactics don't get rewarded, and also quite lame when description doesn't mesh up with the effects you can have on it.
Except that is a dangerous road to walk down - writing an elaborate combat subsystem for a single monster seems, in my mind, a waste of time and effort and an invitation for slowing down combat and potentially penalizing groups that don't think of the exact right tactic. It also means having to address a variety of questions - can PCs target the demons all the time? Can they hit them with area effects? How many demons need to be killed? If the characters spend four actions to kill all the demons and weaken the dragon, wouldn't those four actions have been better spent killing the dragon itself?
The rules already have a solid, comprehensive framework for rewarding creative tactics - page 42. They don't attempt to define a specific list of creative actions and how they work, since that would both be a pain to create, and actively prevent other creative solutions not thought up by the designers.
Personally, I like this dragon as is, and would certainly let my PCs try stunts to damage the demons while they fight the dragon. I find its nature as a non-elite interesting but not unreasonable, given the explanation of how tortured and tormented it has been (and I would probably play up that element in the description.) I
am slightly concerned by its offense level as a non-elite: between Breath Weapon, Pain Frenzy and Spikes of Pain, these guys seem to produce a lot more damage than feels appropriate, and a group of such creatures is straying into dangerous territory as far as balance goes. I do like the solution proposed of a scaling AC if it is upgraded to Elite, as a
simple mechanical way to represent the demons crawling about it.
Easy to run, distinct and memorable in nature and appearance, this is the sort of creature I am very glad to see. The balance issue is my only concern, and that might be more of a hypothetical issue than one that would actually arise in gameplay.