Here's an interesting snippet from a Bangkok Post article about Thai Studies. I have see a couple of crowned Buddha images (hokan shakka) in Japan. They are unusual in that while Sakyamuni sits in the classic meditation position, his left hand is on top of his right hand.

"Thus, Thai Studies can be seen as a study of the particular in a world that upholds more liberating universal values. My own research on Thai Buddha images illustrates this point. For nothing exhibits the quality of Thai-ness better than images of the standing Buddha with both hands offering protection and dressed in the regalia of Thai kings. So much so that they represent the images of the Sayamnikaya sect, or Tantric Theravada, which modern Western scholars of Thai Buddhism refer to as the National Religion. Such images were made by the first three kings of the Chakri dynasty for their own benefit, or who transferred the merit gained by making the images to their immediate family. These images, as you see them in the Convocation Hall of the Temple of the Emerald Buddha, are covered in gold plate and are richly decorated with enamelling and precious stones. The practice of making these images stopped when King Mongkut decided to have life-like images of his predecessors made instead of crowned Buddhas.

However, he died before the project could begin. So his son and heir, King Chulalongkorn, had the project completed in 1873, as it had been his father's wish. From then on, no more images of the Buddha dressed in royal attire were made.

Until the present reign when the government, led by prime minister Chuan Leekpai, presented His Majesty with such an image on the auspicious occasion of his 5th cycle birthday in 1999; that image is now kept in the Hor Phra Monthian Tham in the Temple of the Emerald Buddha."

the U Thong and Ayutthaya periods saw most of the crowned Buddha images cast in bronze - the crown is held to be a symbol of kingship - thus deifying to some degree the Kings of the period. Overlooked is the fact that devas are supposed to have crowns, jewels and ornaments by nature and these fade in lustre as the deva approaches death. This style of Buddha image is in stark contrast to earlier images from India and Thailand, which tended towards simplicity - due in many cases to the roughness of laterite that the images were carved from.

When I studied Buddhist art history, I was taught that regalia on Buddha images is primarily intended as a reference to the Buddha's royal lineage, and secondarily to signify his status ranking above even Indra, the king of the Hindu pantheon, in the cosmos.

Crowned Buddhas are common in all SE Asia Buddhist sculpture, depending on the period. The Post quote seems to imply it was peculiarly Thai.