Assuming one has the requisite language skills, visa/citizenship, and finishes with a decent class rank at a top-25 school, how possible is it to -- at some point, not straight out of school -- get a legal job overseas?

Very possible. Depending on where you would like to work, many countries are itching to bring Americans on board. For example, I have friends practicing in the United Arab Emirates (UAE). They all came from great firms here in the U.S. (Florida, New York, Michigan) and they were all making very good money. They took jobs in the UAE, some right out of school and some with 3-5 years of experience, and they couldn't be happier. They've all said the same thing, the foreign countries love American lawyers, and they compensate them accordingly.

While I certainly believe that every individual should speak at least 2-3 language, as an aside, there's often no language prerequisite as most of the business is conducted in English.