Why is a 17lb cat ok in the passenger area but not a 22lb cat?
What is the heaviest inanimate object permitted to be loose (unstrapped, unenclosed) in a plane cabin?
A pet cage with a heavy object inside it is an inanimate object. That it contains a living thing is beside the point from a physics perspective.
British Airways’ policy on hand luggage is any weight is fine, as long as you can lift it into the overhead bins without assistance.
The heaviest Atlas Stone lift is 560lbs, which seems like a reasonable answer to your question.
It looks like Atlas Stones are generally mold-poured concrete, so you'd have to pour concrete into a rectangular shape.
What is the maximum mass (in pounds) of concrete that can be poured into a carryon luggage rectangle compliant with BA policy?
EDIT: 51 pounds, because that's the BA published limit on carryons. So, 16"x12"x3.451" using 133 lbs/ft^2 pre-mixed concrete, best case scenario. Make sure to take the handle's weight into account.
https://www.britishairways.com/en-us/information/baggage-ess...
https://www.calculator.net/concrete-calculator.html?slableng...