Dude... Your bike looks SO good like that and so compact being a full suspension. I'm a little jealous!!!
those fenders really set it off!
I think a derailleur tensioner would be good. I have been thinking about the tension problem very deeply and I want it to work well. I'm going to take one of your side pics and photoshop a ruler and protracter on there to see exactly what the movement will be like. If you were able to make a shift kit style bottom bracket jackshaft but without a freewheel, it might be possible to utilize the original chain position the rear triangle was designed with. What do you think? Cause then the chain slack due to suspension movement would be minimal, and maybe it would be possible to do it without a tensioner and allow engine braking.
Another thought is making a tensioner that is somehow connected by a lever to the swing arm, like an adjustable suspension link in a car... A follower... That would adjust tension automatically (mechanically) following the suspension movement. I'm going to think hard about this, because this could be big for our needs in the motorized bicycle world.
Here is a slack calculation I posted in another thread. This is a low level calc for pivot and shafts all in line and not taking sprocket sizes into account... If it even matters.