I really dislike this idea, leaves way too much room for abuse. It would allow a malicious user to exempt their channel from defcon and exploit it somehow. However, I'll meet you half way by suggesting the exact reverse, defcon exceptions. This would allow admins to exempt channels from defcon mass modes.
The thing is, anytime you go into a lockdown in anything, it's best to use deny,allow rules instead of an allow,deny rule. It's much easier to manage for everyone.