If I were to fly with a vanilla PPL holder and we did some basic under-the-hood handling work (or flew in IMC enroute) would they be able to log PUT instrument time?
You are legally entitled to give the instruction (as is any pilot) and the PPL holder is entitled to record time spent flying by sole reference to instruments in their logbook, which is nothing more than a personal record. However, the instruction cannot be counted towards the requirements for the issue of an instrument rating as you are not qualified in accordance with JAR-FCL 1.330(d)