I believe assets has to be attached to a game component somehow, otherwise it will not be included in the built game. On the component you are creating, you could create an array of textures, and assign the possible textures to the elements in the array.

In C#:

Texture2D Textures[];

Texture2D FindTexture(string name)
{
    foreach (Texture2D tex in Textures)
    {
        if (tex.name == name)
        {
            return tex;
        }
    } 
    return null;
}

If you don't want to attach the textures to the component, an alternative is to add the textures to a resource folder, and Resource.Load to load the texture