Hmm... I dunno if this helps or not, but I did stubble on a (non-Star Wars) custom with apple barrel acrylic paint once, by dabbing on little spots with a toothpick and then scratching them off to little specks carefully. I used a pin to do some of the scratching and to apply a few tiny dots here and there also.
Of course, an obvious effect of this is that if you handle the figure's head much the stubble will "reduce" because the rest of it chips off too, but the stubble's stayed on the figure I did it on a good 2 years now.
These other methods might work a little better though, especially depending on the size of your figure. The figure I did this on was 1:6 scale (12"), so...