So I went by a dealership last week to order parts for that and see what happens. I have the caps and overflow tank now, and at least I mostly understand how the upgraded system works.
So in the new system, the plastic tank is actually part of the full-pressure system. The replacement cap (in our current cap location on the water outlet) is just a seal for the upper edge, it doesn't have a spring in it at all, meaning the little line to the "overflow" is full-time full-pressure. The bottle's obviously reinforced to handle it. It has a normal spring pressure cap and the outlet from there is what overflows to the little tube that runs down to ground. There's also a 3rd line on the bottle, coming off the bottom, that's meant to T into the heater line. The bottle still has min/max lines.
So I guess the basic idea here is: the whole system including the plastic bottle runs full pressure, but the bottle has a giant air bubble at the top of it (also at pressure!), and any air in the system ends up there because it's the highest point. When coolant needs to expand or contract, it does so through both the little line from the top of the water outlet to the top of the plastic tank, and the slightly larger line from the bottom of the plastic tank to the heater lines.
With the bottle itself and both new caps (the bottle doesn't come with the pressure cap btw, separate parts), the only thing that's really missing for doing the upgrade is that, on 2012+, there's an extra fork on that hard heater line coming out the side of the engine, specifically to go to the bottom of the plastic tank. It's where you see #22 in this pic (also note #21, which is for the factory oil cooler):
You'll notice that new stubby bit on the heater is facing a certain direction, seems like perhaps so the normal flow of the heater line tends to draw from the tank rather than push liquid into it? I could hook up the system using the (#27) straight bit that went to the throttle bodies before. It's slightly smaller diameter and not angled-out like the other, but it might work. Either that or upgrade that whole heater-pipe section as well, but then I have to deal with capping off the new outlet for the factory oil cooler.