I'm none the wiser after reading the IUPAC manual: "The value of E measured when the left-hand electrode is at virtual equilibrium, and hence acting as a reference electrode, may be called 'the potential of the (right-hand) electrode with respect to the (left- hand) reference electrode. This terminology should not be taken as implying that single electrode potentials are measurable."
Looking at it another way, the 'rule of thumb' is still partially correct. The average oxidation state of carbons in glucose is zero. In carbon dioxide, the oxidation state of carbon is +4. If applied strictly to the RHE, it wouldn't work because the H+ is obviously higher than zero in O2. I'm not convinced that H+, H2O |O2 | Pt is a viable solution for the RHE even though the species with the highest oxidation state is next to the salt bridge.
As you can see, I don't really understand this question either! It sounds like we need an expert!