Thinking aloud!
Different airframes have different engines, some having a choice of different types
Each individual engine type is type tested
I recall from books/documentaries that each individual volcano can produce differing ash compositions that possibly cannot be replicated.
So if airframe type A with engines type B were to fly through the ash cloud of volcano X, all that would prove is that combination would/would not work
So to prove it was safe every airframe type with every engine type would have to fly through the ash cloud to ensure flight safety
i.e. you would need to type test every engine type with every volcano!
Of course that means multiple flights and a proportional increase in the risk of an unnacceptable event.
So again would you as an individual or member of an organisation be willing to accept the responsibility if one of these test flights ended in tragedy. I don't mean litigation but the knowledge that you may have been resposible for loss of life?
Sorry but I thought the aviation industry had the highest safety standards!
EDIT Though I realise that some of the go fly in the ash school may not be directly involved in the industry