Open only for PP/II/CG really would not work unless the entire way instancing, and balancing are handled is changed, they make it truly cross platform and not just multi platform, and reduce the effect cargo delivery has on the outcome of PP/CG/II. You generally do far more for your faction in PP/CG by delivering cargo than you do by trying, and utterly failing, to blockade.
What is the actual goal? Bringing more people into open, or just eliminating the effect Solo/PG players have on the BGS?
The very idea that open only will bring a lot more people into open rather than them simply playing another game, is likely wishful thinking. I suspect many, if not most of the Solo/PG players would simply not play anymore. If it is to eliminate their effect on CG/PP, well blockades are pretty useless in my opinion. You can't stop more cargo by killing cargo ships from other factions, (even if you could identify them when it comes to CGs), than you could have delivered yourself.
Anecdotal evidence, ignore if you like:
The last time I dealt with chain interdiction, I tied up 3 opposing ships for over 15 minutes and still managed to make my delivery. Meanwhile who knows how many slid through unopposed while they were playing with me. Even if they had toasted me on the first try, it would have been at least 5 minutes spent with me while who knows how many tonnes slipped through. A half dozen ships in super cruise does not an effective blockade make. 60 ships spread over 20 instances even less so. Those ships would do way better for their faction delivering cargo than trying to blockade.
Granted, I was in a heavily engineered Cutter, and not a Type 9 or the like. Lots of shield and fairly fast. Faster than the Annie one of them was using at least.