Very interesting topic!
I have been giving this problem a lot of time and my current thinking is this: We could attribute Embodied & Operational Scope 3 carbon depending on your influence over each project. In the same way the influence of a designer changes from Concept to Handover, the chain of custody for the carbon necessitates that we each consider our part and account for it. If a designer has the opportunity to reduce carbon when they have the influence to do so but chooses not to, then that should be reflected in an increase in their scope 3 accounting.
As the degree of influence will vary greatly depending on scope/discipline etc I agree that co-operation is key. I would love to see perhaps LEED, BREEAM or failing that AIA, RIBA, BSRIA, RICs, ICE, ASHRAE, CIBSE, FIDIC etc get into this and overlay a notional percentage into their stages/systems/contracts broken down by stakeholder. Has anyone else come across this kind of approach? I am sure we are not the first to think of this. Thanks, Iain