Let me say that i love this mod idea, a Total War game that would cover both Ceylon and South India would be really awesome
But, there was somebody here not too long ago who proposed an 'India: Total War' that would cover the rise of the Mughals in 16th century India....sadly not many people seemed interested in a pre-Empire Total War era India mod for Medieval 2, and im afraid that if thats the case a mod centered around Sri Lanka would have an even lesser chance...most people (Americans anyway) probably cant even point out Sri Lanka on a map (sadly)
This being said, let me ask you a question:
What are your modding skills? Do you have any skills in any software (photoshop, gimp, 3ds max, milkshape)? OR are you more interested in researching? (in which case ideally youd be quite knowledgable about Sri Lankan history

)
The reason i ask is because a mod im working on right now (Nusantara Total War - a Southeast Asian modification) includes a Sri Lankan unit....i know its not much but eventually we plan on including the Arakan faction in Burma with its capital at Mrauk U, a 'sister city' of Trincomalee (indeed the kings of Arakan and Kandy shared a common ancestry, and many Sri Lankans lived in Mrauk U and many Rakhine peoples in Trincomalee

)
So if you are knowledgable at all about Mrauk U/16th century Ceylonian history i encourage you to send me a PM, we could use your help even if just for the creation of the 1 Sri Lankan unit
-Pazu