If I remember correctly Kannon derives from the Chinese Guanyin which in turn derives from the Indian Avalokiteshwara. Interestingly Avalokiteskwaras play a major part in both Buddhism and Hinduism. In Hinduism they are merely called Iswara and associated with Shiva, while in Mahayana Buddhism they became known as Avalokiteshwara.
http://en.wikipedia.org/wiki/Avalokite%C5%9BvaraI have seen Guanyin and Avalokiteshwara Statues with Bindi on them. A lot of Hindu deities ended up in the Buddhist pantheon in slightly different/even identical roles so maybe that is where the bindi came from. Bindi is also found in the Hindu/Buddhist temples of south east Asia as well as ancient Vietnam(Champa people).
During the Tang Dynasty, which btw was one of the most culturally open and progressive of the Chinese dynasties..and one which had extensive relations with India, the use of Bindi became fashionable amongst the aristocracy and later even amongst common people, and it is during this period one starts to see a profusion of Avalokiteshwara, other deity sculptures/paintings with Bindi on them.
PS: the godchecker website is pretty cool
http://www.godchecker.com/pantheon/japanese-mythology.php?deity=KANNON
She's the Japanese version of China's GUAN-YIN, who is really a female manifestation of India's AVALOKITESVARA.
Being a BOSATSU, she spends her time tending to tormented souls who haven't yet seen the light. If you're lost in Japan and looking for Nirvana, KANNON will put you straight. If anyone can, KANNON can.
btw Kannon is pretty cool. I just realized that the Anime Kanon is probably based on her name! One learns new things every day! :D
K&R