Only hard-shell panniers would work as deck expanders but even then, not very well. You cant lash items to the edge of the pannier platform, only to the rack and you cant open the panniers when carrying a load on top.
The 3-part panniers that drape over the rack are no good for carrying big loads, the top compartment usually does not have stiffening over the top of the side compartments (else it would obstruct their opening).

When my 2 cloth panniers are fully loaded, they do offer some support for big, lightweight items such as 6-packs of kitchen paper roll.