Hi, Rob:
Are you talking about the impedance to ground? Will a PWM approach work? Will a regular PWM voltage output not work?
I don't know which chip you are using or what you're interfacing to, but if the standard PWM does not get you what you electrically need (varying impedance), you could try modulating the pin impedance in a very crude fashion.
When a pin is an input, it has a very high impedance. When a pin outputs a zero, it has a very low impedance to ground. If the device won't mind the impedance varying between the two extremes in a modulated fashion, you could bit bang the duty cycle.
By putting a zero in the pin's data register and modulating the pin's direction register, you could effect an 'average' impedance over time. You could solicit the help of a timer to bit-bang it, but it may not be very fast.
If you need to vary the impedance to Vcc, simply change the zero in the pin's data register to a one.
