I like the idea because it would 100% give JB more stability, but getting the actual role as Head Guard raises some red flags to me. I definitely can see people taking this and abusing it right away. The CT having pretty much full control for half the round is a lot of power.
Personally I like the idea of CT's having access to just an !order command where says the ct's get muted for 5 seconds and the T's for 10 seconds which gives plenty of time to give an actual order and the CT's having a respite from the constant mic spam that ensues on jailbreak. With that command perhaps like a 30-second cooldown. To relate this back to the original idea, perhaps at the beginning of the round the guard that takes !hg would have access to that command so he can give his orders. Even with a headguard system when there are 30+ players on the headguard talking wouldn't be heard with some of the mic spam that happens on JB which is why I suggest the !order for command for him.
My last thought is really farfetched, but I'll say it anyways. Perhaps, someone that wants to be able to use !hg needs to take a rules quiz before they have access. I imagined this by using a menu (similar to the one that appears when voting) that's just some basic rules of being a ct and maybe a couple of questions could be a scenario and you need to select the proper way of handling the situation. If this was implemented in any way I would think just 5-10 questions that could easily be answered in a minute or two.