How to secure a fully carbon (aero) bike when washing/repairing on a stand?
The seatpost is a fairly sturdy carbon tube, as it has to withstand the mass of the rider on one and of it and some significant bending forces.

Also, the seatpost has to be clamped by the frame, so it can take some clamping force. 

For cleaning, you should not have to clamp the seatpost particularly hard, just enough to support the bike. 

I note that Simon Richardson of GCN clamps a Ridley bike on its carbon seatpost in this video.