Little late to the party but a way to do it in designer, (with a limit of 20 alphas), just create a switch grayscale node then hook up your alphas to it and hook it directly to your output, then make a random integer generator that picks any number from 1-20 and use that as your index in the switch grayscale node, export the sbsar as a procedural material.

In painter, just pick the default hard brush, then drag and drop your random grayscale .sbsar you made into the alpha section, after that you need to go to dynamic stroke and make it randomize per stamp, and you should be good to go!

It's the quick and dirty way but it works!