BIL, Dielectric Breakdown, Shrink Wrap, and Safety. Is there a resource for this information?
I don't want to pretend that I have experience with very high voltages, but I can give you a couple pointers.  

"My question is, can that number (dielectric breakdown) be used to reliably assess safe insulation levels for such high energy pulses, assuming that the pulse voltage is lower than the breakdown?"
- I would say, use that number only as reference point, but aim for at least double the calculated insulation. Personally, if I were expecting a 600V peak in my cables, I would make sure their insulation is capable of withstanding AT LEAST 1000V.
 Safety is a tricky "business", and whatever you calculate, the real world is full of unexpected surprises. You could have a great insulation, but some sharp object may puncture or thin it and allow for those short pulses to eventually cause its breakdown, meltdown and finally a powerful arc or a short circuit, or simply an exposure of people to the high voltage(s). This is a very realistic scenario. A high temperature source nearby may weaken/soften the insulation, or cause it become brittle, creating cracks through which the electric field can puncture, or conductive liquid may seep into. Lightning or similar unexpected transients could superimpose and create much higher voltage peaks.
Anything of such nature can happen, just use your imagination. A safety "padding" of 50% can never be considered "over the top". Not even 66-75% (meaning the insulation capable of withstanding 3-4 times the maximum voltage.
The higher the voltage, the more unexpected paths that can be formed for a current flow. It all depends on how safe and confident in your insulation do you want to feel.
 I hope that I have somewhat answered your question or helped in your decision-making process.