There seems to be some confusion here about the "R" terminal.
First, to answer the OP's question, no, you can not power anything with 12 volts from this terminal.
"R" does not stand for "Regulator," it stands for "Relay." The 12 volts on this terminal does not come from the regulator, nor is it used to control the alternator. Rather, it goes the other way -- this signal is a half-wave rectified DC output from the alternator stator, whose sole purpose is to communicate whether or not the alternator is producing power. Because it is a single tap on one diode, the output voltage is approximately
half of the alternator's rated voltage (so roughly 12 volts for a 24-volt alternator), however the signal is not straight DC -- it's half a sine wave.
Note that there is generally no connection between this terminal and the regulator. The only thing that should be connected to this terminal is the coil of a 12-volt relay. That relay's contacts are then generally used to control two things -- the passenger A/C and heater blowers, and the "Not Gen," "No Charge," or similarly labeled tell-tale on the dash. The idea here is that the energy-hungry blowers will not run unless the alternator is putting out charge.
The way the regulator is involved here is that if the regulator stops calling for charge for whatever reason (such as it is controlled by an air pressure switch, or there is an alternate charge source such as a battery charger supplied by a generator bringing the voltage up above the regulator's set point), then the voltage on this terminal will drop to zero. This will be true even though the batteries (or alternate charge source) are still maintaining the voltage at the alternator's main output terminal at 24 volts.