Forest Pro can use any object as custom object, including mrProxy and convert the trees to instances using "Forest Tools" plugin. This way, it can work with all types of proxies. We have succesfully tested it with VRay, Mental Ray and Maxwell Render. But as noted before, this must be done in two steps: 1. create Forest using billboards, 2. convert to instances (there is a small demostrative video on the website).
The problem with Mental Ray is that in our tests mrProxy didn't seem to work very well. We got some crashes trying to render scenes that were using large amounts of objects (one mrProxy by tree). I'm not sure what was the problem, however we tested it before Max2009 SP1, may be this issue was fixed yet. VRay and Maxwell were working fine instead... we will repeat the tests with mrProxy, i will post here the results soon.
About the private forums: well, initially the Forest Pro boards were designed as a place for our registered users to post questions, and get support. But you are right, now there are a lot of information that may be useful for all users, so i just opened them for public access. By now, on read only mode, because we like to differentiate the posts written by Pro users.
If you have any question or suggestion about the software or the website, please tell me.