ARe DC systems included in the arc-flash studies ? 125vdc control batteries as well as UPS systems ? Anyone know ?

The guideline doesn't go into DC and Single Phase faults, but my opinion is that the intent is to cover all electrical systems. There are other risks associated with DC battery faults, the case of the battery could actually explode. Also, since the battery is its own energy source, the fault may occur in a location that would not trip upstream over current devices. Because of this I am not sure you can even calculate incident energy because there could be no t value. I also don't know if it is right or wrong, but in the field I have been using a PPE rating of 2*. But we have changed the 2* rating from the bee keeper hood, to the face sock and face shield. I feel face protection is important because should a rupture occur there would be acid thrown out.

You can cap the energy released from the battery to the Ah rating x voltage: when it's empty, there's no energy left to sustain an arc. That means a lot of energy released, especially in a battery bank.

If you want to use the time, the available power can be calculated from the internal resistance of the battery and it's voltage (V^2/R). The initial power is likely high, which means the IE for any meaningful working distance will go over 40 cal/cm^2 very quickly. Of course the power will tend to go down with time (voltage drops), but there's still a tremendous amount of energy in a battery bank.

Then there are all the problems with FR garments which don't resist to acid, leakages, the small 1/4-20 bolts to which you must connect a lug with Class 0 gloves, etc.