I found some info related to what appear to be early use of shields by Samurai at My Armory. I am trying to get verification and more information if I can.
It is an image of a Samurai shield from the Heian period (794 to 1185). Here is an illustration of Emperor Jinmu accompanied with a foot soldier carrying a shield.http://horse.shrine.net/samurai/jinmu.html
It was stated that the Japanese kept the "tate", a type of pavise, and it seems the "kabuto"or the "jingasa" (equivalent of a kettle hat) would be used as a buckler when one was left with a short sword or dagger. It was also mentioned that Yagyu Shingan Ryu still practices those techniques.
Here are some photos of renactors carrying those shields:http://www.pbase.com/joanseeuw/image/41052063http://www.pbase.com/joanseeuw/image/41052065
It would be great if anyone can add to this info.