The CAAP says "unique situations" but the CAR just says "having regard to the circumstances of the case". All I'm arguing is that it is permissible to do it in a different aircraft (as in your example) if the reviewer approves.
I agree that doing it in a different type is likely to take longer (and probably be more difficult), but overall it may provide more value to the reviewee and may also give the reviewer a better idea of their abilities.
In many ways it makes more sense to do it in the most complex aircraft you expect to fly in the near future. If you have been flying a C152 but are about to purchase a Warrior, it would make sense to do the AFR in a Warrior, even though you could argue that you are entitled to do it in the 152.