After much searching I am back where I have started. Nothing I found explained how to find the proper combination of hidden_resources to make a unit recruitable in the provinces of my choice.

However by logical deduction I came up with a theory on how I can accomplish this. Take Gawjam~Kimbrioz and Gawjam~Skandzawarjoz for instance, let us say that I wish to make a unit recruitable in those two settlements but nowhere else. I find the provinces in descr_regions and then find the relevent resources for the two provinces:

NW, C, y1, n2, y4, y8

and

NW, D, y1, n2, n4, n8,

respectively. Now I don't have a clue what this means, but when I take the two resources that the two have in common (y1 and n2) and give them both to my new unit as hidden_resources, then my unit will be recruitable only in those two provinces.

It sounds worse than it did five minutes ago when I thought it up. There are probably other provinces with the same two resources come to think of it. Would someone mind explaining to me how this system works?