A minority government would be formed under an agreement that the Linke would elect Schroeder and back him in the future in case it comes to a no confidence vote. They could do this without actually being a part of the government. Of course Schroeder's government would immediately fall as soon as the Linke would withdraw their support, but that's in no way different from how a governing coalition is dependent on all parties in the coalition. An SPD minority government "tolerated" by the PDS (the predecessor of the Linke) has been tried at the state level (in Sachsen-Anhalt) and there's no reason it couldn't be done at the federal level. I really don't see any difference between Germany and other parliamentary democracies in this regard. But, like I said, it ain't gonna happen.

If you can't convince them, confuse them. (Harry S. Truman)