Hiding the built-in screen from the system

The screen in my 27-inch, late 2012 iMac developed problems which make it unusable. I don't have the bread to replace the screen so I'm using an external monitor set as master.

Since the startup and safe mode screens only show on the built-in, this is less than ideal.

I've tried disconnecting the two internal screen cables but unless the video cable is attached, the heat sensor is disconnected and fan runs at full speed.

With the video cable attached and the power cable disconnected, the system thinks the built-in is still active and I can't see the startup or safe mode screens.

My questions are these:

Is there a practical way to cause the system not to see the built-in screen during startup without detaching the internal video cable?

Or,

Is there a way to allow the fan to detect the screen heat sensor, with both the internal video and power cables detached (or partially detached)?

I don't mind cutting through something on the screen cables if necessary. The screen is junk anyway.