That sounds like something a Warlock with a master who pays attention to his antics can achieve, too. Modify the 'available spell' list as needed and you are good to go.
Think about it though in the context of the setting. The Sorcerer Kings are defilers, and their most powerful spells can be fueled using non plant based life. Otherwise they are still constrained by the limits of the setting. How would they channel arcane power to warlocks if they can't even do it themselves.
And even then assuming they are patrons and warlocks are still subject to defling laws and rules why would they create a heap of defilers when they rule the last few fertile lands themselves? They themselves have to be careful with their defiling and they have to use trees of life to offset that otherwise their own cities would be ash piles. How do they create a tree of life if they themselves are defilers (defilers can't create them BTW, preservers can)? Well they can't I would assume they get their Templars to do it for them (or pressure elemental clerics and/or Druids into it).
They are also not that big on sharing knowledge either in a setting where they have suppressed both magic and the written word so I don't really see them acting as patrons to warlocks either. The Templars are a fairly corrupt lot, generally evil if they were arcane casters most of them would be defilers and that would create additional strain on the Kings (Queens) tree of life reserve.
Sure the SK's have a few pet defilers (probably a handful) they can't actually create a whole bureaucracy of arcane casters and have it make sense in the setting. That and pact magic is actually clerical on this world. The SKs can't even channel spell power to their Templars (its stolen off the Elemental planes). I suppose technically a warlock would not have to be a defiler but then you have a whole bureaucracy of preservers serving defilers whose main job description is to suppress other preservers where a main theme of the setting is the preserver vs defiler thing.
That and adding more arcane casters waters down the whole concept of the setting where magic while powerful is harder than other worlds. Even the 1/3rd casters don't really belong IMHO, if you want an Arcane Trickster multiclass a wizard/rogue. Logically, thematically, mechanically warlocks make no sense in Darksun especially as servants of the SK especially en masse. Just because you can add more arcane casters to Darksun doesn't mean you should and even the Sorcerer makes no sense on Athas unless you refluff the class and even then most of the Sorcerer archetypes still don't make sense to include on Athas (eg Divine Soul, Dragon Sorcerer, the Storm one etc). The wild Sorcerer at a stretch maybe and that is assuming you refluff it.
The preserver/defiler duality and the reasons behind it along with the Templars are major elements of the Darksun setting. They back pedalled on a lot of the changes 4E made to FR, the same reasons mostly apply to Darksun and the designers who made those stupid decisions lost there jobs over it it seems (the ones who pushed 4E hard).
As I said why would the SKs have a whole bureaucracy of defilers or preservers working for them either way its a bad idea.