Very serious! you cannot have a policy where a young inexperienced CC can be placed doing flight deck duties after a few weeks training to become a CC and no real knowledge of his /her background.
surely it would require a level of training and background with the company of possibly 3 years to be a safe option.
The best security should include a degree of randomness, as a rigid prescribed system can be subverted by any decent problem solver. So why not leave it to the senior CA? Or the Captain could choose:- "could XXX come in while I take a leak?"
That way you get someone who is both available and suitable.
Your way would be a rostering nightmare. If there Must be a half-bar CA on every flight, in the front galley, what happens if they take ill and there is no qualified standby? Do you cancel the flight?
Then think about "How could a person with ill intent USE this system?"