I think those are actually shields. Both a square shield and a round buckler were used at least by the Thai and the Burmese, and I think these guys are supposed to be Siamese troops. These shield look like both, but I am guessing they are supposed to be square shields with a round decoration on them.
The dominant religion in the Khmer Empire up to this period was Vaishnavite Hinduism, but Buddhism was gaining ground. Among the Burmese and Tai (including the Siamese), Buddhism was the religion that replace animism. In terms of material culture, there was an overwhelming Indian influence throughout the region, which in fact are referred to as the "Indianized" cultures of SEA, in distinction to the "Sinocised" culture of most of Vietnam.
Armor was little used in continental SEA. When used, it was most commonly just a padded jacket or leather brigandine. Higher-ranked people did wear mail or semi-plate armor in Indian styles. It looks like the cavalry guys in the Angkor relief are wearing either decorated cloth coats, or brigandine.
You have the same problem with SEA art as you have with Indian art -- it is overwhelmingly depicting religious themes, and so is both highly stylized and standardized. It is hard to discover historical or regional clues in the art, because the same styles were strongly maintained across time and region. Interestingly, it is mostly in the depiction of the "bad guys," like the army of Mara and these two ceramic raksha that started the thread, that you see more freedom of style and often recognizable ethnic and national dfress and weaponry. One that comes to mind is the inclusion in a Thai fresco of a Dutch soldier with plumed hat, long coat, and musket, among the demonic hordes attempting to stop Gautama's enlightenment.
