I added a couple Haradrim units to ye olde Haradrim Palace. They can be built and work fine. I have them listed in SkirmishAIData, they're in the command sets for the Palace correctly and all. Each is even just a mildly re-tooled and renamed copy of Haradrim archers and Lancers.
The AI just ... never builds them. And I cannot figure out why. All relevant files are attached below. I just haven't been able to figure out what I'm missing. I'm sure it's something obvious somewhere.