I have never had this need but I spent some time thinking about the issue before going to sleep last night.

I think any kind of contraption like rope ladders etc in the water will be hard to use. Rope ladders themself are hard to climb.
From the climbing world there is a thing called a "Jumar" or in its modern form: Petzl Ascender. With a classic technical climbing setup with one Ascender and one "basic" rope lock, you could use the same technique as long as you have one 9mm line leading into the water.
It is quite a setup to get to know, and best if rigged before the capsize. But it should allow you to not use significant power in your upper body and put the load on your legs instead.
Look at the picture and you can see that he is using a sling down to his foot to "climb" the line.

Just a suggestion if you have to rely on your lower body to get onboard the boat after a capsize. Might be unsafe as well if you find yourself trapped under a speeding boat after righting..
http://www.petzl.com/en/outdoor/ascenders