Wood and plastic composite

If by plastic composites you mean a laminate like melamine, then from an FSC CoC standpoint, you can't exclude it from consideration because it contains kraft paper. Many major manufacturers of high-pressure laminates now have FSC-certified offerings, so it should be possible to source FSC-certified laminate panels that combine these materials with FSC-certified substrates like particleboard or MDF.

I'm generally familiar with this product.

I believe the correct approach would be to treat the decking as an "assembly" per the LEED Reference Guide, and to calculate the amount of "new wood" as a percentage of the total weight (or volume), and then to multiply this by the total value as invoiced to the project contractor.

So if the composite decking is 30% wood by weight (or volume) and the rest is plastic resin, and the total invoice is $100,000, and the decking is not FSC certified, then you would add $30K to the total amount of new wood used and hope that at least 50% of that total is FSC.

Or if, as seems likely, you can show that the wood used in the composite decking is not "new wood" but rather is pre-consumer recycled content (i.e. manufacturing by-products), then you should be able to exclude it from MRc7 calculations altogether and instead factor it into MRc4 calculations if that is a credit you are pursuing.