I don't know much about signaling, but I thought the whole point was to spend money on nothing. It won't work if the money goes somewhere useful.

For example, let's say that a Rolex costs $4000, and that you could buy an equally-beautiful watch for $500. You're spending $3500 for nothing. If we can believe MacAskill's book Doing Good Better, $3500 is about what it takes to save a life in Africa. If there were a $4000 watch that included both the $500 watch and a saved life in Africa, and showed it conspicuously, I don't know that it would work as a signal. I'm wealthy enough to buy a $4000 watch that saves a life, but not a $4000 watch that doesn't. Only a truly wealthy person would buy the Rolex, and that's enough to ruin the save-a-life's watch as a signaling object. (Put another way, anything that I would buy can't possibly be a signal of wealth.)

In the diamond example, I think your idea would ruin both real diamonds and synthetic diamonds, since no one would want to appear to be a jerk by buying a real diamond, nor want to appear poor by making good use of their money with a synthetic diamond (with 90% going to good causes).

It's like smoking cigars with $100 bills. The whole point is that you get nothing out of it.