It appears to me that by the time double spending is detected you have two problems. Somebody has already lost their money and it is already too late. I am confident that an attack would be executed in seconds with the initial spend and the blocks in the pool (s) wouldn't be dispensed until the new chain was longer than the original ... so by the time it is detected it is too late.
You can't do it instantly, the attacker have to recreate all the confirmed blocks. If the sum is so large, that you are afraid of double spend - than wait for enough blocks. E.g. 6 blocks = 2 hours with 50% of network.
In my opinion, monitoring for such block forks is much more important, than trying to impose cartel agreement of not having more than 50% on pool operators. If we could have such warning in clients, than it would be even better. Maybe such monitoring service should provide the API and charge a small use fee?