Does anyone know if the IEEE 1584 arcing current calculations factor in the X/R ratio and asymmetry? Does the degree of offset from the DC component impact the arc flash energy?

I don't think this was factored in. I heard the budget for the original testing was on the low side. I would think if there is an asymmetrical offset that the thermal energy would be this same since the total area under the curve remains constant. Just shifted with more above the horizontal axis than below. One thing that I would be curious about is the higher the X/R, the greater the peak current which produces the maximum magnetic force. This might need considered in the future to evaluate forces, blast etc.

Robert Wilkins has written a paper that includes X/R into the calculations. So far his method has not been taken up with the 1584, although his has a basis in the physics of the event.
The papers are "Improved Method for Arc Flash Hazard Analysis", by R. Wilkins, M. Allison, and M. Lang. Also R. Wilkins’ follow up paper on "Simple Improved Equations for Arc Flash Hazard Analysis."