How to superimpose a synchronization pulse on a 12V DC power line
The way that I would approach this is to have the output of your isolated DC-DC converter feed the end device via a diode.  There is a bulk storage capacitor after the diode that feeds the end device.  You also have something that detects when the input voltage to the diode drops.
Obviously, the diode and bulk storage capacitor are located at the end device.
It's far easier to detect a voltage drop than it is to try and force sufficient current into the DC line to cause a voltage rise.
You would have something like an opto-isolator at the supply end that either interrupts the output of the DC-DC converter for s specific period or causes a voltage drop.  Either method can work but detecting a complete absence of voltage at the end device is probably easier.
The above is a simple approach that is already in common use in several types of devices.  But you are going to have to provide many more details before anything concrete can be suggested.