There has in fact been an effective "Champion or Champions" (6N champions v 3N Champions) match several times in the last 10 years. In each case, the teams were their respective champions at that time;
2009 South Africa v Ireland (Croke Park, Nov 28, 2009 - Ireland won 15-10)
2008 New Zealand v Wales (Millenium Stadium, 22 Nov 2008 - New Zealand won 29-9)
2007 no match
2006 New Zealand v France (Lyon, 11 Nov 2006 - New Zealand won 47-3)
2005 New Zeakand v Wales (Millenium Stadium, 05 Nov 2005 - New Zealand won 41-3)
2004 no match
2003 New Zealand v England (Carisbrook, 12 Jun 2004 - New Zealand won 36-3)
2002 New Zealand V France (Stade do France, 16 Nov 2002 - Drawn 20 all)
2001 Australia v England (Twickenham, 10 Nov 2001 - England won 21-15)
2000 Australia v England (Twickenham, 18 Nov 2000 - England won 22-19)
NH 3 wins
SH 4 wins
DRAWS 1
NO MATCH 2
All that needs to happen is for the iRB to make sure that the current 6N Champions and 3N Champions are scheduled to play on either the November or June tour immediately following the 6N Championship, and to market it as a Champion of Champions match and perhaps make it the last match of the tours. No extra match would be required, and it would put give some "edge" to the game.