If you guys do work on this, send me an email (email is in the profile).
Halting is not the right way to run an exchange, because it creates "air pockets" of liquidity given sufficiently sophisticated traders. This is what lead to the "flash crash" last year: high-frequency traders didn't want to be caught with illiquid positions in the event of a halt, so they sold as quickly as they could, making a halt more likely. It creates a situation quite similar to a bank run.
(Instead, you might want the people who organize the exchange to also do some market-making, and to stabilize prices in order to grow volume, leading to a more profitable business overall.)
For sufficiently fungible products, the "Market Maker as Exchange" model works really well. What you'd basically have is high spreads for retail investors, and lower spreads for institutional investors. So your consumer-facing company would make a spread and then flatten its exposure by trading with other market-makers.
This gives the "Exchange" more flexibility than Mt. Gox has. It can offer more competitive prices when it's worthwhile, and pull back if trading with retail customers gives it too much net exposure. That's better than having a fixed fee for matching people with market-makers.
Halting is not the right way to run an exchange, because it creates "air pockets" of liquidity given sufficiently sophisticated traders. This is what lead to the "flash crash" last year: high-frequency traders didn't want to be caught with illiquid positions in the event of a halt, so they sold as quickly as they could, making a halt more likely. It creates a situation quite similar to a bank run.
(Instead, you might want the people who organize the exchange to also do some market-making, and to stabilize prices in order to grow volume, leading to a more profitable business overall.)