Optocoupler: decoupling circuit parts sharing same ground and power line
If the step converter is also isolated to the level of insulation required adn is short circuit protected  by design then it would not defeat the isolation of the optocoupler.

The bigger question is why would generated voltage become shorted and why would that damage a controller if one was careful to protect it without insulation but rather just high resistance and clamp protection.

Obviously the choice is yours as your needs are unknown.

However when I designed a 2nd source supply for proposal at Lexmark printers, when this model was being prioduced at 1k units a day, it required no feedback isolation from HV output to 5V PWM and 24Vdc input , it generated regulated 3 non-isolated outputs with sweeps from -10kV to 15kV and used a string of 10M resistors to convert to the 0 to 5V range and was fully protected from damage with diodes and measured by differential input with HV return line sharing leakage to stay within the CM input range.  This was the Vcharge transfer controller to drum for laser printing and was accurate to 1%.

There are still many details to consider and a few obstacles to avoid for crosstalk of CM noise, inbalanced CM to DM pickup but there is no problem per se with what you suggested, except it may not be necessary.