It has to act as a supply for a high side driver. So it has to be able to handle the output "ground" wildly changing. Most DC-DCs are completely insufficent for that. One that does work is something like the VLA106-15151, which is $18 in quantity 12. The isolation rating is for 2500v for 1 minute, and in real life it can do that all day long:
Digi-Key - VLA106-15151-ND (Manufacturer - VLA106-15151)
You really have to get the DC-DC, take it apart, and see how they do the isolation of the coils. Lots of chinese DC-DCs just wind the secondary right on top of the primary, and the isolation just comes from the enamel on the wires.
1000v isolation in real life translates to blowing up at around 200v according to Otmar. He had to move up to 2500-3000v isolation because DC-DC components kept failing that were rated for 1000v isolation.