Well, I just tried over_voltage=0xEA60 (+0.060V offset) with a 1.7 GHz overclock and it works as expected:
pi@azazel:~ $ vcgencmd measure_volts
volt=0.9163V
pi@azazel:~ $ vcgencmd measure_clock arm
frequency(48)=1700419968
So the answer is yes, you can set enter the voltage offset in microvolts by specifying a hexadecimal value. The warranty bit was not tripped (the revision number is still c03112).
I have not found how to enter a negative offset this way and I'm not willing to experiment any further at this point. Any comments adding more information would be appreciated.