If I understand you correctly, you want the light bar to come on with the high beams, but only when you also have the light bar switch on at the same time. I think your best bet may be the 5thGenDIY harness with the high-beam trigger. You can route the trigger (which is active-high, as in "on" is 12V) through any on-off switch, then to the relay. That way your switch interrupts or allows the high-beam trigger to determine whether it turns on your light bar. You are correct in that the BCM pins operate by grounding (called active-low) to turn something on.
If you find yourself in Houston, I have a diode with your name on it.