The problem with using an adjustible voltage regulator is most of them don't allow you to adjust the voltage that low. I know the LM317 only goes down to 1.25V. I like to use zener diodes as a cheap method to limit voltage, but that won't work for this low a voltage either. Unless you use a 1.98V zener then run through 2 standard diodes (each droping the voltage 0.7V) That would put you right at 0.58V. That's not software though

What size capacitor did you try with your PWM and what frequency did you use?