Switching high voltage in a buck regulator from an awkward ground reference
As you mentioned, the controller "ground" and the V- can be the input voltage, 300V, apart. That happens when the switch is off and the inductor current is flowing.

Any direct-coupled switch (MOSFET or BJT) design referenced to V- is likely to be quite inefficient. That 300V is going to appear across something across the drive circuit when the switch is off. For example, while the gate is protected by the zener, most of that 300V is going to appear across the PNP or the resistor. With your current schematic, it would be very difficult to pick the resistors such that the switch would operate effectively.

So if you want to use a V- referenced power switch scheme, you probably need to use some form of isolated drive (opto-coupler type or other form of isolated IC driver).

Without an answer from the manufacturer, I think it is safe to assume you cannot subject any pins of the TL494 much more than 1 diode drop below its ground. A simple way to isolate the emitter output of TL494 from negative voltage is to add a PNP in the common base configuration. Base to "ground" (with a current limit resistor), emitter to TL494 output, collector as the new output.

But why not use an offline LED driver ICs. These issues would be taken care of. It would require much less components therefore simpler and cheaper. And since it is purposely designed, probably more efficient and reliable.