I'm not talking about the physiological match between the donor and the receiver, I am talking about the circumstances that would trigger such a forced donation being an acceptable option in your mind.
Without a physiological match the point becomes moot. "Sole possible donor" is quite different from "a possible donor".
As long as there is another option that's where I'd start.
But in an attempt to address your point. Suppose Bob demonstrably caused Joe's catastrophic kidney failure. Maybe Bob poisoned Joe or something. Bob is the sole possible donor. Under such bizarre circumstances what I'd probably be inclined towards would be something like this:
Bob is under indictment for attempted murder. As long as Joe stays alive, the charge is only attempted murder. Donating a kidney will keep the charge the same. Refusing to do so ups the charge to premeditated murder.
Bob goes to the darkest, ugliest, prison the judicial system can find, where he'll stay until Joe gets a kidney. If Joe dies for lack of a kidney, then Bob stays in that prison until he dies.
Voila!
No force. Personal responsibility and choice.
Tom