FFFFFFF...... thats easy.
Think of it like a hinge, and you can position it where you want two objects to rotate. Just put it where you want it to rotate, and if you dont want it to rotate, don't bother calcuating the midpoint, just put it as one of the env_obj positions.
I'd be better and easier to explain if i could meet your on IRC, just highlight me in the #modteam channel.