I can see it now........
A crowd has gathered to watch two great contestants pit their wits and skill against one another, but then, out of the blue, someone whispers out to another "I bet you 10 gold that the blue guy is going to kick red's butt, what do you think?"
Inevitably, some will whisper an answer "10? Ha! I bet you 100gp that your wrong."
Then someone overhears and says "Hey, I'll take that bet. Let's see you put your gold where your mouth is bigshot."
As the game goes on, someone just blurts out "Man, these guys are good, this has been going for five whole minutes, I wish i had something to eat."
Someone will say back "I'll sell you a bag of assorted nuts for a few gold."
This could lead to unknown amounts of under-the-table style betting, with money holders taking a small commission, possible payoffs to take dives. Even Food vendors walking up and down the rows offering beer and nuts. Maybe high-end business talks from The mayor of Hlint with a visiting politician that can be...accidently listened to.
The possibilties are endless.
Also, you would have to make sure no interference from viewers. Can't have an invisible spectator quietly make an opposing monster drop dead in a mass of confusion, leaving his favored player to coast into a victory. you could...i don't know, give all players True Sight while just in the box, that way, they won't miss a beat.(like what you did with the road systems)
Thoughts?