It has a sensor that disallows running it with the case off. Disabling the sensor would void the warranty. Not saying nobody will do it, because I'm quite certain someone will (or already has?).
But you'll probably have to actually spring for your _own_ MacPro to mod the sensor.
Thermal design might rely on a laminar air flow inside the cylinder and taking the case off could result in increased tempatures. Of course we will only know if someone tries it and reports the measured temperatures.
But you'll probably have to actually spring for your _own_ MacPro to mod the sensor.