Diode Drop vs Open Drain Output Tolerance
I don't think the diodes would matter for that limit. When the output is open, the diode drops will be 0V and the pin will see the full 12V. Transistor voltage limits are there to prevent the transistor from being ripped apart by a strong electric field. It doesn't take much current to do that.

I'm not sure whether a clamp diode or a parallel resistor on the input would help, either. If it draws enough current to create significant diode drops, the LEDs will turn on. If it doesn't, the pin voltage will go up and exceed the limit. If the LED current exceeds the bias current for the driver IC, Vcc might start to rise unless your regulator can sink current. (Most won't.)