I like the idea, it would help out people who aren't in a clan be able to defend in at least a slightly more organized fashion.
You can't honestly expect a bunch of random people on siege to be able to communicate well together and devise plans, especially without voice com. Chat is also very easy to miss while in the middle of a fight. Yes, it's the defender's job to guard the flag and keep an eye on it, but without a clan structure and voice com that's not so simple.