If you go as part of the Peace Corps or other similar organization, they take care of the paperwork in order to enable you to practice medicine in the specific country you are going to. This carries the implicit agreement that your stay is temporary (although as long as 2-3 years, it is still considered temporary) and that you are not going to "open up shop" so to speak, outside the scopes of the organization you work for. Other kind of medical work that doesn't require the validation of your license and obtaining work permits are the literally "in and out" projects, such as Project Smile or Orbis. Depending on the country, short term medical missions must have the staff working under a local doctor's license but, again, if you go as part of an organized operation this is not your concern as the organization takes care of the details.
You may want to check out any medical oportunities with the UN, although many times they will use doctors from a variety of organizations such as Medicins sans Frontiers. The CDC may be another possibility, although overseas assignments are short term, epidemiological rather than clinical and a small fraction of your job overall. If you have a strong religious affiliation, you may want to check with your church for any opportunities overseas. All of the above obviously implies that you will NOT be working in a developed country.
Also keep in mind that there is plenty of positions avaliable in the US if your goal is to practice underserved medicine, check with the National Health Service. And you can still volunteer to go on short overseas missions on occasion.
Take the people who go overseas for medical school. Most places will not offer them the possibility to stay there if they were unsuccessfull in returning to the US. Even though license revalidation is not an issue in such case, limited work opportunities, visas and work permits are.
By the way, medical school in Europe is almost free for nationals of each particular country. There usually is a university tax, which may amount to one to 2 thousand dollars a year plus the expenses for your books and equipment, but that is about it. Not even at the cheapest state school in the US you would be paying so little in tuition. So their doctors don't graduate carrying the burden of a huge student loan debt.