The maximum voltage on the RDCM and RVOUT pins isn't specified in the LT3750's datasheet/
In the LT3751 I can see a 60V clamping diode, but it's not the same in the LT3750/
So, what is it for the LT3750?
Accordance to DS the current into these pins limited to ±1mA. Can you recalculate current to the pin based on your max voltage?
The max current can be 700uA if I let this pin with ~240V and a 365kohm resistor, but I'm not sure if the component won't be destroyed..