Interesting idea, hide a few kbytes per day of important transmissions that really need to the kept private in a gigabit per second flood of child pornography. Not very practical though.
And it must be thus. If the system is actually secure the illegal activity is going to be there. And good luck in scaling the more legit activity to a point where it even matches the yuck traffic when latencies are measured in seconds. And good luck getting enough exit nodes to mitigate that when anyone smart enough to install one and with good enough connectivity to be able to contribute one realizes that almost all of the traffic coming in or out is going to be illegal.