It turns out PWM is very easy. PWM (Pulse Width Modulation) refers to hardware on the IOIO that generates regular pulses of varying "on" durations. This can be used to simulate analog voltages, drive servos, or rapidly pulse the digital inputs on a motor controller to control motor speed. It's this last use that is my immediate goal.
In reading the IOIO wiki at https://github.com/ytai/ioio/wiki/PWM-Output I learned that the yellow status LED on my board is hooked to a PWM output. If you apply PWM to an LED, it dims as you change the duty cycle. Perfect. My little client program already has a JSlider component that has a value of 0-100 that's sent over the network to the rover as part of the command string. It was really easy to set up and use that value in my IOIO loop.
//initialize the pin as a PWM output
private PwmOutput led_;
//in the IOIO loop, set the duty cycle on that pin according to the value of the JSlider. Scale it to 0-1.
float dc = (float) (servoPanValue/100.0);
This results in the LED being dimmed to varying brightness (off to 100%) as you move the slider. Too cool. Now I need to update my motor driver code to set up the pins as PWM outputs rather than pure digital IO to pulse the input pins on the motor controller board.