That said - I have an idea. And this needs to evolve into something more solid.
What if transactions are posted in encrypted format. Something along the lines of TITAN for the orderbook.
Let's say we have the following time line
At Block T, A has a confirmed order to buy listed in the order book. B wants to sell at the confirmed price A has listed in the order book.
B submits an encrypted pending order, which gets confirmed in Block T+1
B then submits the code to decrypt the pending order at Block T+2, which confirms and executes it, unless A managed to confirm order cancellation in Block T + 1.
Downside is that it now takes 2 confirmations instead of 1 for transaction to occur.
Also make orders extremely cheap, however extremely expensive to cancel. (i.e. they should be allowed, but this will reduce the bloat in the chain, as well as drive away those buying/selling non-stop)
UPDATE: mixed up A & B. Should be good now.