Fixing the leak is the best solution. Much easier and more realistic with a team of one (yourself) than it is with more than one.
While we can all probably give examples from personal experience equivalent to the real estate agent leaking the minimum price, my personal favorite resulted in the current border of California. Ever wonder why the West Coast border between the U.S. and Mexico is between San Diego and Tijuana? At the end of the war that resulted in this border, there were American troops occupying Mexico City. Why draw the line there? Turns out that the U.S. negotiators had a list of must-gets and nice-to-haves. Alta California and Colorado country were must-gets. Baja was viewed as a desert waste and a nice-to-have. The list got leaked to the Mexican delegation by an American personally acquainted with one of the Mexican negotiators, so the Mexican team dug in their heels and wouldn’t give up Baja. Point: keep negotiation teams small. Fewer leak-points. One (yourself) is best, because only then do negotiator interests exactly correspond with YOUR interests.
Is this question really so hard? Remind me never to hide from Nazis at your house!
First off, Kant’s philosophy was criticized on exactly these grounds, i.e. that by his system, when the authorities come to your door to look for a friend you’re harboring, you should turn him in. I briefly scanned for clever Kant references (e.g. “introduce the brownshirts to your strangely-named cat, Egorial Imperative”) but found none. Kant clarified that he did not think it immoral to lie to authorities looking to execute your friend.
The larger issue here is the purpose of rationality. We start in medias res and reason is a tool to help us navigate the world better. I imagine that none of us have a commitment to rationality for its own sake, but rather support a clearer world-view out of some initial kind self-interest. Consequently I’m a-okay with engaging in the Dark Arts in cases where even my basic interest (my own and my friend’s continued survival) and that of another party totally diverge. Otherwise the joke about the engineer (or atheist) and the guillotine isn’t really a joke.
Often the long-term best strategies in game theory are irrational in the short-term; as in, games of chicken, or in punishing wrongdoers even though the cost of punishment is more than letting them off.