Well, at least I can document this 
for anyone else considering a similar effort. Reading Ken Shirriff's blog post regarding Arduino PWM, the Arduino's default pre-scaler value of 64 and default clock speed of 16 MHz yield a PWM frequency of:
16 MHz / 64 / 256 = 976.5625 Hz
Desiring a cutoff frequency for the RC filter lower than 976 Hz, I chose an arbitrary 47 uF capacitor and 35 ohm resistor to yield a corner frequency of 97 Hz.
Does this make sense, or is that placing the cutoff frequency too low? :-/