How fast do you need to change bars? It takes like five minutes to unscrew the four bolts in a stem faceplace entirely, replace the bar, and screw them back in.

You CAN cut carbon bars, there's cut marks and all, just use a hacksaw with the correct blade, the bars will have instructions telling you the correct way.

Finally, I'm not sure about your QR bolt idea, it might be ok for flat bars but I wouldn't trust any kind of quick release on any other kind of bars, not to mention the contradiction of a lightweight stem with whatever kind of QR system you were looking at. A seat post has almost no twisting forces, and if it comes loose you won't be happy, but you also won't go smashing into the front of your bike and the ground. Take note of how many people run standard bolt seat post binders.

In addition, I wouldn't use any kind of QR device with anything carbon. The force you'll be putting on the bar (or seatpost) would be pretty random and hard to control, and especially if you're trying to switch them out fast you might run into over tightening or under tightening issues. The margin for error on carbon is much smaller than steel or aluminum.

Back to the real topic, what are you actually trying to do? Why do you need to change bars as fast as possible? It seems like there'd be any number of better options. Maybe just invest in a nice allen wrench in the size of your stem and keep the bolts nicely greased so you can spin the bolts out/in faster? Open faceplate stems are designed for the faceplate to come off entirely for a reason, it's just easier, loosening them and sliding the bars through seems like too much effort, too limiting in bar covering, and could possibly end up marring the carbon fiber's clear coat.