I've not seen Foleypro's plugin so I can't really comment on that but I'm assuming its based on the following principal.
One way to do it would be to render a half dozen or so different looking trees arranging them sporattically, then do a mask render of the same scene, save the two as .jpg. Do this 2 or 3 times with different looking trees each time. Apply these three set's of two .jpg's as a texture to three seperate 2D planes then group. You could then use the multi replicate tool to multiply the number of trees in your scene. This would populate your forrest quite nicely and cut down a lot on the poly's but not sure how it would look when used in an animation. Never really played around with Bryce's animation tools.