"Is it illegal to deliberately approach at a speed less than Vref or 1.3Vs? Does that risk CASA legal action if caught in the act? Just wondering."
Depends entirely on the opinion of the FOI of the day.
Whether its "legal", "illegal", safe or unsafe has no bearing on CAsA's actions, the FOI's opinion is the prime consideration.
CAsA are not subject to the law as we know it. If they determine that a legal case may be flimsy, the always have "Administrative" action to fall back on.
A simple example is Mr Carmody's actions in regard to Angel Flight, in his own words "He does it because he can" like little Johny sitting in the garden pulling wings off butterflies.
What motivates him to do as he does is another matter....and No I'm not suggesting for an instant that brown paper bags are involved!!
All passing strange but.