There was a period in my life when I had access to bandsaws and welders and ... and probably would've tackled something like you're contemplating. Now that I see what goes into making a good commercial roaster, am older and less motivated, I'd leave all that to the guys that make them for a living.
My first thought is that you'd cut a section of the keg out lengthwise so it could sit over a manifold w/ some gas jets, mount the roasting drum inside with a shaft on the back and some sort of funnel shaped opening on the front of the drum, so that the beans would stay inside the drum when it's horizontal, but you could tip the whole set up up and off the base w/ the flames to dump the beans.
Then you'd have to devise some sort of blower for exhaust and airflow.
As your ideas come together, you'd have to ask yourself whether or not the whole project has any advantages over a BBQ grill using the drum you're going to make.
