Unfortunately past tense - no longer have the tank.
The osmolator has an optional kalk dispenser add on which enables you to top up the sump water with kalkwasser rather than straight RO water. Worked very well, apart from when kalkwasser seeped back down the line and blew the pump. A friend of mine worked out a way to avoid that, but I can't remember how he did it off the top of my head.
Probably simpler that way (but as you found out, not much!). Loved the constant top-up it gave me, rather than the whole, topping up by eye thing I used to do before getting it.
Yes, no doubt it's better than topping off by hand.
I'd wondered about the Tunze kalk chamber. It looked like it relies on the flow of water to stir the kalk into the water, and I thought the kalk needed to be actively stirred.