Not sure these would be considered keys but they are definitely extremely tough to find signed. I believe this list will be similar to those mentioned in the "toughest cards possible" thread that was started a while back...
http://www.net54baseball.com/showthread.php?t=180372
1986 Topps Doug Flynn
1986 Topps/Donruss Ramon Romero
1987 Topps Ricky Wright
1989 Fleer/Score Donnie Moore
2001 Brian Cole (any issue)
2002 Topps Total Darryl Kile
2003 Topps Total Dernell Stenson
2009 UD Nick Adenhart
2014 Topps Heritage Minors Oscar Taveras (may not even exist)