What are “best practice design principles”? Do they have a BS EN number?
A temporary sign is either mounted on A-frames or, less often, “proper” posts. If it’s the latter and the posts used to mount a particular sign were not passively safe, how could you not protect them with VRS and perhaps more importantly, defend such a decision in court? As you say, the duration the sign might be present for is irrelevant.
It’s not a case of policy, surely it’s a case of applying the relevant standard to the installation? Allowing that A frames have never been crash tested as far as I’m aware, although they are rather flimsy things.
It may be that the “will not be used” statement harks back to the days when passively safe posts were new and viewed as being overly expensive, so you wouldn’t use them in a temporary situation, as the argument went in some quarters. Such an attitude is, I feel, rather outdated these days.