So, Haqq can have a Tuareg Doctor,, and his TO gives him access to Hidden Deployment. How does it work if I want to link a Nasmat to him and deploy him in hidden deployment??
I don't think there is anything in the rule that mandates that the controller must be on the table. When deploying the Nasmat, you declare that it is linked to a trooper not yet on the table and the Nasmat will be Disconnected until that trooper is put on the table.
The only obligation is to have the controler deployed So engineer/doctor can have a ghost servant on the table if they are in hidden deployement (it only will be disconnected, as @Mahtamori said) On the other side, engineer/doctor using AD cannot have "standard" ghost servant waiting for them on the table. If they use AD, they need an AD ghost servant
The only obligations are these: REQUIREMENTS Only troopers possessing the Doctor (any Level) or Engineer Special Skills can enlist G: Servant troopers in their Army List. The player must identify which trooper each G: Servant trooper is linked to during deployment. They cannot be used by any other trooper. The user of this Special Skill and its Doctor/Engineer must be in the same Combat Group, where both of them count as only one trooper. Be in the list and define who operates who.
The FAQ tells us it needs to be 'deployed at the same time their Doctor/Engineer is deployed'. A model in Hidden Deployment is deployed. Whereas in AD they're only deployed later.
So yes you can do it, but since AD Docs can't deploy the regular Nasmat your opponent will know that you have a HD Tuareg Doc+.