This is a fun topic to google- so far I found an article about Cetewayo, who ran in a $13,000 claimer, and went on to win a G1 at age 8!
http://sports.espn.go.com/sports/hor...ory?id=1341466
And this article from 1993, about Jerkins, says in 1973, he took two cheap claimers and won G1s with both of them, but it didn't say who they were:
http://query.nytimes.com/gst/fullpag...pagewanted=all