I'm having a 1/4 rai pond dug soon in Korat and was looking for suggestions on something easy to plant around the edges to keep the soil in place for the eventual rainy season.

 

The soil is a rocky mix, but seems to have enough clay to hold water.  Eventually I'll put in trees and other plants, but for now I'm guessing spreading around some kind of local grass seed would be best? 

You could try Ubon forage seeds for some grass varieties.

 

The traditional way in Thailand was to plant หญ้าแฝก Yar Fark known for holding soil in place, but now not easy to find, you need to ask around, have seen it growing wild.

Or easier use Yar Con in Thai, or Para Grass, you will know this grass, it grows in wet diches next to roads etc., can be grown from stems, but be wear it can take off and may need regular cutting.

As FJ said, seed from Ubon Forage seed would do the job, pick one of the hardier verities, unlike Para Grass most verities are shorter growing, easier to control, but be wear not all verities like waterlogging, that if the land lies wet.