Is it possible to calculate the 'overall' redox potential of a system?
Is it possible to calculate the 'overall' redox potential of a system?


No, it is impossible. You can have a situation where a cellular compartment contains a high ratio of $\ce{NAD+:NADH}$ (i.e. favoring oxidation) and a low ratio of $\ce{NADP+:NADPH}$ (i.e favoring reduction). This is advantageous for the cell because it can run oxidation of some metabolites in parallel with reduction of other metabolites, just by employing enzymes with distinct co-factor requirements.

This is different from pH. A solution is either acidic or basic, not both. The difference arises because acid/base reactions are usually fast, and water acts as acid or base, "communicating" acid/base status across the solution.