Balance of power is more than just who's most powerful...that's just ranking, and they have that in EU2, or...Risk, for that matter, just count the bloody pieces. Balance of power takes into account coalitions of states. For instance, even though France is the strongest country, a coalition of Prussia and England is even more powerful - the Seven Years' War - even though neither Prussia nor England are more powerful than France herself.
With good luck, the AI should be able to deal with the balance of power itself. The system Metternich devised, for its own ends of balance, turned out to use what was natural in each participants' own drives and such. The balance was made between each of the conflicting interests. In theory, the balance of power should arise with a smart AI. All the same, it'd be nice to have a measure of international strengths and stuff - a raw measure for this purpose - aside from mere score.