Sounds like you've thought it through Mike. If your paint layer just hits fibers and doesn't span the open pore space (i.e. you can blow through the paint layer) you should be fine. If the paint coat is continuous, and you can't blow through it, I don't think it will work as effectively on frequencies above a few hundred hertz.
Please let us know how it works out, I haven't seen anyone try to paint rock wool in this context . There is the theory, and then there is the real world.
Sometimes you just need to try stuff. If the paint has too large an impact, you can always flip the panels so the paint is on the back side of the panels, against the plywood, then the paint won't have any significant impact.
I assume this will be outdoors.