The idea doing it on the barbecue is that the heat from directly underneath will heat the beer up quickly and steam the inside with glorious flavour, this chickeny beery broth will also drip down onto the coals and the rising steam will further flavour the chicken.
You must have a lidded barbecue for best results though.
You don't need to brine if you're doing it on a decent grill.
I've tried using a longer can directly in the coals before but it did dry out the bottom half of the chicken. I think you're better off using the rack on the lowest setting and a wire coathanger to balance it.