I was discussing this idea with a mate a few months ago.
Well you should have done it.
This is great. I love the game, from whenever I saw it in that show.
A lot of things can be done on/with the blockchain, people just need to let their imagination loose.
Especially feed based betting with full built-in escrow of the funds... I'm waiting for it to take off. Honestly. In Counterparty, anyone can create their own feed for the price of a BTC transaction fee, and anyone else can bet arbitrary XCP amounts which are stuck on the blockchain until the bet is resolved or expires.
Example; "Will BTC go over $1000 this year? y=1 n=2" with a value of -1 (so it doesn't resolve a bet), and then you post on the same feed with the resolution of the bet at the end of the year (either 1 or 2) or if it reaches $1000.
You can bet on ANYTHING this way. People just need to know that you are a reliable feed operator and wont report the wrong thing (you can charge a % fee for your services). CfDs are also supported, but there is a bug to iron out before they can go online again.
On github, I suggested implementing betting on block hashes directly (trustfree) as well, without a feed! This may appeal to the people who find RPS to be too much of a hassle..
For example (I am in no way shape or form connected to these sites, but you can check that they are reliable directly on the blockchain
http://blockscan.com ):
http://betxcp.comhttp://xbet.io