Tim, would not a small zener diode be the best way to go? I do not think there would be any lost energy like with a resistor.
That is how I dropped the 12 volts to 6 volts for my 46 ford instrument panel. No heat sink or anything.
I wouldn't recommend it for anything other than a low power load (less than a watt). The zener will dissapate anything over the specified reverse bias voltage (so for a 12Volt part with a 28.8 volt system voltage, the zener will dissapate 16.8 volts)
If the series load is 5Watts at 12volts (5W / 12V = 0.417A), the zener will need to dissapate: 16.8V x 0.417A = 7Watts.
If you recall the V-Reg approach, the power disapated by the V-reg was also 7Watts, so you're not really saving anything here - the drawback is finding a zener which can support that power (the limit for common zeners is 5Watts).
In the end, a zener isn't as suitable for voltage regulation of power loads as a task-designed power supply or voltage regulator. Zeners are good for low-power voltage references (less than 5mA of current) for larger voltage regulation devices.