It might be better for the actual IAU to have an amateur branch, thus using the administrative resources they already have, and not setting up an entirely new organisation. I have had some contact with the IAU Assistant General Secretary, who is British. A problem with the ‘amateur IAU’ idea today is that this might be seen to cut-across the IAU’s new, and major, Citizen Science agenda. ‘Citizen Science’ was not a term that existed in the 1960s-80s. The landscape has changed.
British Astronomical Association
Supporting amateur astronomers since 1890