For the size and weight I can understand how its hard to find a such motor.
Have you thought about making it a sort of wormgear actuator instead? It will take up a bit of space but surely you can manage to compact it, so that it fits into the frame when the arm us down.
In config as a geared brushless motor that should be a decent setup i think.
You will need a sturdy harness though 
thats quite an amount of forces at the base