Thank you for the prompt response as always, Unikron. You are my hero!
Could this approach be simplified if I create two separate tk2dSprite objects with separate SpriteCollection (possibly with two cameras)?
If this theory is valid, I guess I just have to wait for the group registration to get my hands on that alpha mask code. I hope 2 cameras won't drop performance significantly on iOS devices.
I'm very devastated by the steep learning curve of shader, especially with Shaderlab with lack of publications related to the topic.