Interesting DD, here some Qs:
1) Will factories be built/run with the foreign investor or the domestic techs? A badly unciv state could lack the ability to build anything except very few types of firm.
2) Will this process involve some basical tech transfer (maybe with a low % of firing, like an invention, to acquire an obsolete master's tech in industry or commerce tree, something like 2 steps behind of the master's current one) or give research/westernize bonus to target? I'm thinking to something like the Western arms trade bonus of EU3.
3) Will the "host" country able to destroy unwanted factories, even with a laissez-faire politic?
I fear that AI foreign capis could spam the target state with lots of factories of their choice, filling the slots with firms a player would not desire and couldn't destroy. On the other hand, a subtle GP human player could move very low profit factories to his spherelings, using them to consume his raw materials surplus while he frees slots for telephones, radios, cars and other stuff that spherelings will no longer have room to produce (once they'll have the techs, of course).