We'll think on your suggestion, but as Rokas said, probably is beyond of Forest Pack purpose.
Physics requires to compute a simulation, so it may work fine with a few objects. But with large distributions, the computation rises up exponentially.
I think that would be very problematic with Forest Pack, which was designed to handle tons of items at realtime.
We have used Physx Painter sometimes, and is very cool. But as expected, too slow for large areas.