Until you find more information on how these attacks work, I'd suggest disabling custom channels. Having custom channels is not worth repeated attacks causing disconnections.
If you know the core well, you MIGHT be able to write something blocking access to custom channels by level, however if people still want to attack the server using that exploit, I'm quite sure they would take the time to level up, just to attack it.
Take a look at the quest, try doing the quest and see if there's any point in the quest if you logged out at a certain point it would force disconnect users, I wouldn't see this to be the case but its worth a try.