hi moms... just came back from Canada and all of us are dealing with jetlag! but we were blessed with our relatively smooth long haul flights so no complain on that.
we had no problem going through immigration in Hong Kong & Canada with a thermos of hot water, pre-boiled water bottle at room temperature and ready to go formula (500ml bottles)... yes i brought everything just in case, but ended up mainly breastfed while on the plane!
from Hong Kong they didn't even ask us to separate the liquid from the carry on while screening but security is definitely more straight while in Canada (for our domestic flights as well as when departing from Vancouver for Hong Kong), everything had to be screen separately and scan with a liquid machine (makes me feel like I'm in CSI!). Also, in Canada they had huge signs saying no thermos flask are allowed beyond security check, but when I asked they said there are exemptions for babies at age 2 or under.
by the way the flight attendants are very helpful in assisting to warm the bottles with a hot water bath while on board, so if you are planning to use ready to go formula no need to worry about bringing hot water on board.