High Voltage (0-600V) DC Voltage Sensing, Shared Ground with Isolated Converter and HV-
The isolated DC-DC you picked has an isolation rating of only 1kV. That's not enough to meet safety standards nor is it in my opinion enough to use on a 600V bus. The idea can work, and you can tie the DC-DC isolated ground to the HV-, but you probably want a converter with "reinforced" isolation of at least 5kV.

You can read a good explanation of the various types of isolation HERE.