Hmm, I think it could work. And it sounds like an interesting addition
But it does depend on people not taking it too seriously. Many of these systems can be 'gamed' but so what? I wouldn't specifically try to outlaw such behaviours until you see them. Do you really think lots of people will drop games on turn 13 just so they can get a cheap point or two? Maybe I am reading the forum wrong but I don't think many will do that. People probably won't take rankings that seriously will they? Do they take the Hall of Fame that seriously?
It would be good to see some form of rankings for the powers. I think it would add to the interest and help to keep people from power picking even more than they do already. QM did a ranking guide a few months back and I am sure you could get a group of people to rank the powers and adjust the rankings as fashions change. That is important too. Remember when Baalz wrote that guide about Maverni? I am sure for a brief period their popularity and perceived strength rose
Until everyone realised it was all just a joke and they really were the pits as originally assumed
You could also consider changing rankings based on settings. My feeling would be that LA ermor is far stronger in games that have more provinces per player and it should gain (as most strong powers benefit) from no diplomacy games where gang ups are far more difficult. Rather than banning such powers or games simply adjust the points.
I think you can and should play against the existing rankings. You simply fix the ranks at the beginning of the game. Regardless of how unrealistic they are at that point, or how good a low ranking player has become points wise by the end. The first few games have everyone the same - on zero. But that's needed anyway as the better players need to wrack up the points to take their rightful place on the higher parts of the ladder.
If you do attempt it I would gather a group of admin people who can help to access the nations, work out the points system and make any needed changes (as there will be if you notice abuses, simple tweaking or if you notice it favours passive turtling or some other tactic you don't think reflects who should do well on the ladder).
I would encourage as many games as possible to be ladder games rather than setting strict criteria for joining. Some very heavily modded or team games should be excluded perhaps, but most should be eligible. That way you will gather up the data you need to make it meaningful all the sooner. You want lots of Ladder games otherwise it becomes irrelevent. I think you will also want to make the points system and any rankings you do fairly flexible. You should get plenty of good and bad feedback and encorporating any constructive advice and criticism should go a long way towards making the Ladder acceptable to most. So be prepared to make changes.
Rather than simply start the ladder I would propose your idea / points system in more detail and ask for comments. Get some feedback, and some forum buy in to the idea, and reveal some of the things you won't have thought of before you start. It would also be a way to get some helpers to administer the ladder in place before you start.