Not sure if the South African regs are based on European or USA? NZ is mainly based on USA system of organisation, details com efrom the depths of history which makes it more than likely RAF-based from waaaaaaaaay back.
New Zealand has a slightly different way of doing things though - the operator says what they want to do and CAA says either, "That's OK" or, "How are you going to ensure the minimum standard is met or exceeded?" It's largely up to each operator to research and design a system that works for them, then it is regulated by the regulator, not dictated by the regulator. Hence we get to say who logs what on our operations, another company may do it slightly differently.
So the PinC of the command practice (as we call ICUS) flight is the right seat 'supervisor', and if that's me and our Ops manual says I need a multi-engine instructor rating (which it does), I log P in C and instruction. The other pilot (doing the flying) logs Command Practice. We count Command Practice as part of the time-on-type P in C towards being able to act as P in C on ATOPs (multi-IFR and VFR charter and Air Ambulance).
General opinion of mine is that if you need an instructor rating to do what you're doing, you can log instruction, with the caveat that the instructor should only be able to log instructional flight time if they are in a position to push the student too far and maintain (or regain) control.
If I couldn't take over I'd basically be a passenger, and wouldn't like to be known as the instructor, but I'd also have to be in a different job so I'd ask the Line Ops Manager!
I think for a proper answer you're going to have to just sit down with some paper and your rules and determine if access to controls is involved, or whether you need an instructor rating to supervise a pilot under supervision. Hooray for law! My favourite
