Ah, the old 7-bit text trick—clever!
Great suggestion. The hard part was finding a way of doing it that maintained some readability (on a Retina screen, at least) at the emoji size. There were various Thank You icons in speech bubbles, but they all became really hard to read when shrunk down. I think this 
works, though I’m open to alternatives if someone is enough of a graphics person to make one more readable at that size.
We could also make it another color if that would help, though the black stands out against the other emoji pretty well.