In the past I travelled a lot with my photo-gear for wildlife photography. My biggest lenses then were a 500/4 and/or a 300/2.8, together with two bodies and several other lenses. I have used a Lowepro Nature Trekker several times but I am sure that will no longer be accepted as carry on luggage. Later my wife and I would both use a Photo Trekker Classic which is about on the limit of carry on with regard to size. The problem is always the weight. Luckily we never had to weigh it. Just act and look like it is very light. Many times at checking in I would take a body and some lenses, converters, flash etc and put it in my coat (wish had very big pockets). After passing the check in, I would put it back in the backpack again. Another thing that helped in the past when flying KLM were "Checked Cabin Luggage" labels. I had some of these already attached to the backpacks, making it seem as if they had just been checked and approved.
About ten years ago, we travelled to South Africa with our two little kids (then 3 and 5). They were entitled to their own carry on as well. So we had the loaded Nature Trekker, a Photo Trekker and two full loaded Pelican 1510 cases as cabin luggage. Each Pelican case weighs about 6 kg empty. Loaded they were close to 15kg each. The total weight of carry on was about 50kg. (Tripod etc was in the checked luggage) Luckily nobody tried to weigh the carry on....