Yes you can carry a loaded crossbow at ready, but it doesn't sound like you can carry a loaded crossbow slung on your back. ITL 115 is silent on the unready part.
Most dungeons are flat-floored, so could crossbows be mounted on wheeled carts with shields on the front, giving the bracing bonus for firing and defense bonus when being fired upon.
A bit munchkinnesque, but there is a Lego precedence.
https://brickset.com/sets/1732-1/Crossbow-Cart
Another question, does the +DX bracing and prone firing adjustment count for increasing the rate of fire for crossbows?