In multiplayer, can a player concede abusively?
The rules say you can concede whenever you want to:


  104.3a A player can concede the game at any time. A player who concedes leaves the game immediately. He or she loses the game.


This is kind of unavoidable, because people have to be able to pick up their cards and leave. And it's certainly possible to use this spitefully, whether via your scenario or another. (It doesn't even have to be something fancy - just dropping out a turn before your inevitable death can swing things.)

Many people adopt house rules in order to curtail that sort of behavior. It's hard to define clearly what is and isn't acceptable, especially since you can be plenty spiteful even without ever conceding, but if you play with people you get along well enough with, you may be able to get away with "don't be a jerk". More broadly, if you do things that make you miserable to play with, you might quickly find yourself not having anyone to play with.

As for tournaments, well, there aren't really serious multiplayer tournaments, and so the Magic Tournament Rules don't have anything explicit about multiplayer. There is potentially commander at Friday Night Magic, and if they're abiding by the rules, they'll have to allow you to concede at any time. But while I don't think the store could actually kick someone out for making a spiteful concession, the same about being pleasant to play with probably applies there: it's a local store, with many of the same people coming repeatedly.