Only way I can think to do it is to animate a single instance/layer with 3D Stroke using a very small gap between Start and End values (Start=1 and End=2 - this will give you a dot or dash) -- animate the offset value. Then, duplicate the layer and change the Start and End to Start=3 and End=4 in the duplicate layer...repeat this for every dash, increasing the Start and End values for each new duplicated layer.
Not very elegant, but it will get you a dotted line.
An easier way to achieve the duplicated layers is to apply Trapcode Echospace to the initial layer and then enter say 50 for instances and .02 for the layer delay.
I should have mentioned this, too... If all you are trying to get is a dotted line (rather than dashed), play with the 'Adjust Step' value under the Advanced tab. Setting this to 1000+ and tweaking your stroke Feather and Thickness can give you nice dotted lines.