This question and answer come from above and Missing Manual:
A product has FSC-certified veneer, but a non-certified core.
Can we prorate the MRc7 contribution of this product based on
the cost of the veneer?
No. If the product is built off-site then the entire assembly (the product
which is shipped to site) must have an FSC label from the manufacturer
of the assembly. Individual components, unless they are shipped to
site and thereby complete the chain, may not contribute towards this
credit. All entities that possess FSC materials until the product reaches
the project site must have a chain-of-custody certification.

The question seems to be directed to fraction use of FSC product in an assembly product consisting of FSC and non-FSC materials. Doesn't this apply to Equation 2 of the LEED Reference guide?USGBC ID#10372 mentions "some fraction of the assembly...as FSC certified" and ID#10296 explains further.