DozyWannabe,
In the Bolt/Kennedy report that was instegated to investigate shortcomings by the Civil Aviation Division in relation to ANZ and the Erebus flights, one of the points made in the Royal Commission Report in para 223e Mahon made an assertion that in his OPINION CAA would have approved a lower MSA for these sightseeing flights. He apparently did not understand that MSA's are set by standards laid down by ICAO, not picked out of the sky by CAA or the Company.
No where have I been able to find any confimation of your statement that CAA approved a descent to 500ft if approved by Ice control. Perhaps you could give me a reference?