There are a lot of uses of one thousand percent that predate and have nothing to do with the McGovern episode  its a general expression denoting beyondthebounds effort or commitment similar to the sports clich he gave  percent on the field  Thats why McGovern said it in the first place  The Village Voice one here does not as I read it carry any McGovern connotation  Spotchecking another of your examples this one that usage doesnt seem to have anything to do with the McGovern sense either