Philip,

Hiring billboards is an expensive way to get people to use your word. Your story has received wide circulation of late, but the etymology for 'quiz' is unknown. New World says: [prob. arbitrary use of Latin quis, "what"].

The billboard story sounds apocraphal to me, but who can say? However, such a stunt should have left some record behind. The earliest citation at MW is dated 1749, and I tend to believe some newspaper account would have recorded such an unusual event. I trust the lexicographer over rumors any day.