Baby seat for stoker's seat post on tandem
I don't know of a child seat which replaces the seat-post and I think it's unlikely that it exists as a product as this would be such an unusual case. The normal Hamax seats which attach to the seat tube will work fine and the design shown here provides some suspension. 



I've ridden the bike like this, without a stoker and it does take some extra care due to the weight high up, especially at low speed and when pushing/parking the bike. In this photo I have the rear brake locked on with a strap, to stop the bike slipping. If you were certain you'd never have a stoker riding then, depending on the frame design, you could probably attach it to the captain's seat tube, but on a tandem it can be helpful to have a bit of weight over the back wheel for traction.

I know that doesn't exactly answer your question, but hopefully it's some help.