Sorry Rob, that's just too sneaky! 
You could capture keys... If I'd catch you doing it in my house, I might be angry 
You could see everything that was written, passwords,... But it is an easy way to do it, so Rob is right.
Adapting your router software is "almost" undoable these days for several reasons I fear, unless you are a trained hacker.
There might be another solution, but you would have to go to another site altogether.
You could use a raspberry pi with V-Lans. This way your rasp would become a router and firewall. You could then write a simple script to track how many times facebook.com was loaded from which computer and sent that data somewhere...
It is still no walk in the park, but it is be doable...