The dimensions quoted by the manufacturer are usually for a fully-packed backpack, including the top part. I suspect that if you leave the top pocket empty, yours will fit within the requirements. Only the length of the long stiff part of the frame is important, and it is likely shorter than the limit.
says, send as check luggage, a small bag (or cardboard box) separately with a few items that could easily be replaced. This makes far more sense to me that buying another bag to carry your bag, and then carrying both for 800 km.
When I have had to check a back pack, I have simply tied up the floppy straps and sent it onto the carrousel uncovered. Nowadays, there is usually a tray for such items, so they are not so likely to get caught up. So far, so good.