Well, I have refined the example a bit, but have a new problem:
Exceptions from the thread...

If I wrap a command in a Runnable, it might throw a (runtime)
exception. Apparently such exceptions are only reported, but not
passed on to the calling thread.

I can catch them using an implementation of ThreadGroup that
implements uncaughtException, but re-throwing does not propagate to
the main thread either.

The code below outputs:
Caught exception - throwing a new one
Command was executed just fine

...where I had hoped it would instead die with an exception before
printing "Command was executed just fine".

-- code --

public class OnTimeoutExecuteCmd implements Command {

   private Command wrappedCmd;

   private Command onTimeoutCmd;

   private long timeoutL;

   public OnTimeoutExecuteCmd(Command wrappedCmd, Command
onTimeoutCmd, long timeoutL) {
       this.wrappedCmd = wrappedCmd;
       this.onTimeoutCmd = onTimeoutCmd;
       this.timeoutL = timeoutL;
   }

   static class OverrideThreadGroup extends ThreadGroup {

       public OverrideThreadGroup() {
           super("Re-throw RTE");
       }

       public void uncaughtException(Thread t, Throwable e) {
           System.out.println("Caught exception - throwing a new
one");
           throw new RuntimeException("Uncaught exception from
thread", e);
       }

   }

   public void execute() {
       Thread cmdThread = new Thread(new
CommandRunnable(wrappedCmd));
       cmdThread.setUncaughtExceptionHandler(new
OverrideThreadGroup());
       cmdThread.start();
       try {
           cmdThread.join(timeoutL);
       } catch (InterruptedException e) {
           throw new RuntimeException(
           "I was interrupted while waiting for encapsulated command
to "
           + "join or timeout to expire");
       }
       if (cmdThread.isAlive()) {
           onTimeoutCmd.execute();
       }
       try {
           cmdThread.join();
       } catch (InterruptedException e) {
           throw new RuntimeException(
               "I was interrupted while waiting for encapsulated
command to join");
       }
   }

}

public class ThrowingCmd implements Command {

   public void execute() {
       System.out.println("Throwing now");
       throw new RuntimeException("I threw in the towel");

   }

}

public class TestOnTimeout {

   public static void main(String[] args) {
       Command ote = new OnTimeoutExecuteCmd(new ThrowingCmd(), new
HelloCmd(), 800L);
       try {
           ote.execute();
           System.out.println("Command was executed just fine");
       } catch (RuntimeException e) {
           System.err.println("Runtime exception from ote");
       }
   }

}