Sounds like it'd be fairly amusing. Can't help but wonder if it'd be a good idea to just go directly at the source and adjust costs instead of finagling with custom sites, though. That would let you be a lot more versatile about where/who is, well, where and doing what.
How would you deal with the freespawn nations, though? Mod in low cost or costless recruitment for 'em? Units with dom or active summoning? Just disable the sad buggers? And those poor underwater nations... give
everything amphibious, maybe, and just sprinkle water provinces around? Also maybe some caves. Somewhere.
Also, flying/teleporting/offensive ritual spells/etc. Honor system, mod out, let the shenanigans flow free, something else? Gods know one of my first thoughts with a blood nation would be to take my massive income, hire up a crapton of toad rain capable casters, and blanket everyone else's provinces with crippling income penalties
Similarly, anything with ready access to pop killing spells would just be, like, "Allz y'all get rekt, lololololol" and then walk into the center after everyone else has lost their entire income base.
E: Though in immediate retrospect, that might be a good argument in favor of cost adjustment instead of anything province based. Less room for chicanery.