The line that draws the sprite selector is in tk2dSpriteEditor.cs
tk2dSpriteGuiUtility.SpriteSelector( targetSprites[0].Collection, targetSprites[0].spriteId, spriteChangedCallbackInstance, null );
You need a pair of tk2dSpriteCollectionData and int spriteId to define a sprite.
When a sprite is changed the callback is called you get the new sprite collection + id, and you just assign it to whatever. You get an additional data parameter (null above) which you can use to index into an array, store pointers, etc.