Hi Peter,
Don't know whether you got any responses. I have been looking through the Juliet Scoble collection in the Alexander Turnbull Library in Wellington. I know there are photos of IA wagons in there.
Juliet was a prolific railway photographer in the period 1971 to 2004 taking approx 65,000 photos of NZR related topics, The majority of which were wagons. Some great photos of every wagon you could imagine. I have created ann Excel spreadsheet catalogue of her collection (just the albums, not individual photos) if you would like a copy let me know.
Regards
Don Wills