Decrease a voltage from 24-27VDC into 18VDC using a 7812 and a bunch of other home supply parts
All 7812 implementations I know of have a current to ground &lt; 10mA. I don't see a reason why that would change when you lift the ground pin using a zener.

However, zener voltage is definitely not constant with temperature. Be sure to take this effect into account and determine whether compensation is required when using a zener diode as a voltage reference.