I'd gotten the impression that the rebellion was already simmering quietly, had been for decades, just waiting, and that the mockingjay had already been chosen as its emblem from the start -- and that's where the pin came from. Nice things, in this world, tend to only exist because someone with influence wanted them to exist. Why would they opt for a pin depicting a mockingjay, a creature the Capitol prefers to ignore?

Haymitch was in on it, sure -- though he had a "screw it, I'll sober up when things start looking serious" attitude toward the situation. Cinna had held his design in reserve until he found someone who could serve as a symbol.