Personally I think it would be a really good idea, but only after they'd passed the test. An expanded version of the existing Pass Plus+ would be the best kind, but only after the pupil has had time to get used to a little more driving. A month or two ought to do it, as they need time to add a little polish to the physical aspects of their skill. They will also have time to take in what they've been taught as, depending on the Motorway and time of day they go on it, they will have to improve their mirror techniques and their forward observation and planning by a long way.
As for the IAM, I first joined in 1975 and left a few months later as I disagreed with some of their techniques. I rejoined in the '80s, left again and finally joined once more in 1998. I've recently allowed my membership to lapse as I felt that some of the letters to their magazine were puerile and really shouldn't have been asked by members of the Institute of Advanced Motorists.
One example read (approximately) "My friend says you can overtake on the left in a one way street. I think you can only overtake on the left if the other driver is turning right. What do you think?"
Also, if ADIs are to take on this role, they really need to want to improve matters on the motorways, not just buy a new conservatory.