Hi Alex,
To be honest, I'm not sure how the implementation of Corona Multi Texmap is made, but for some reason Forest Pack can't access all its values during the render time (most probably it's designed in a very specific way just for Corona's purposes).
I would suggest you to use the Render To Texture tool and bake the Corona_Diffuse Texture Element, which you can easily loaded as a Bitmap / Corona Bitmap map and drive the Get Color from Map - As texture on Surface feature. Let me mention, that "Get color from map->As Texture on Surface" works only at Item level (it's not compatible with tint by Element).
Best regards,