You could use that 12v switched supply (YE/BK) to trigger a relay. That relay would need a fused source of 12v itself, to distibute to all your accessories you want switched, so use a big enough wire for power and ground, and an appropriate sized fuse.
A lot of guys do that so all the accessories get shut off with the ignition. I've seen Power outlets / cig lighters set up this way, so they never drain the battery. Heck, you could add another fuse panel and use it to power all the stuff you want, and that single relay would make sure they all get shut off with the key.