Hello Mark
regarding:
"It is my understanding that the BADI allows you to replace WWI - where ever it is used in the standard.
Therefore the BADI would need to be called during report distribution (aka report shipment) as well."
Based on what I can see I have doubts in context of report distribution.
You need to find a solution for the "final report". Anything what I can see in the BADI:
yes there seems to be a suport for CG54/cg50 but only for "display" (and not distribute)
But I am in the same situation like you: Never have had the need to analyise this BADI deeper and sorry to say WWI IS A BIG USEFUL TOOL
(for SAP SD: clearly other tools are much butter (smartform, sapscript etc.)
C.B.