Bad idea. I personally know a guy who was illegally kidnapped by the US government from Ecuador with the assistance of local police. Snowden will regret this decision.
The fact this is not going off the top news makes it much harder - in all aspects (the tactic operation is just one aspect to such act).
You can't come forward against the world's most powerful intelligence agencies and be completely free from risk because they're such powerful adversaries. No one can meaningfully oppose them. If they want to get you, they'll get you in time.
This happened just two years ago. Palestinian engineer, traveling in Ukraine, was captured by unknown people, and was found in Israel prison week later.
The US government has all the time in the world and in months/years from now when nobody remembers his name there will be an opportunity for a extradition to be done quietly.
So really it depends where you are and what you're willing to do.
The appeals process that would go through a system with the power to matter would be through the US courts and would almost certainly be unsuccessful, see U.S. v. Alvarez-Machain, 504 US 655 (1992) [1].
> Is there some court by the United Nations that is mediating such cases even if there is an extreme power imbalance between both nations?
A case could be filed in the International Court of Justice by the offended country against the US alleging violation of a treaty or some other element of international law within the competence of the Court, but for consideration of how much that would be worth even if the offended country won the case, consider Nicaragua v. United States [1].
Maybe this is the plan.
It's not as though you get to practice at this.