Probably would have gotten to this sooner if I hadn't immediately put it into storage after my initial review. Anyway, some pics of the arm construction.
The shoulder armor just clips on, so it's easy to remove. The upper arm is held together by three screws.
The "T" at the joint slots into the front and back halves of the upper arm. With the screw hole right next to the joint there isn't as much room in there to mod as I'd prefer. Almost no extra space within the elbow joint itself either. Given how easy it would have been to design in a swivel below the bicep, I'm not sure why they didn't. I think it'd be possible to add a swivel, but I'm not going to make the attempt until I figure out what I want to do with the figure as a whole.