Agree on the tipping point however, I don't believe they would call for US assistance. I've been to a few Iranian boards to read things from their perspective. They don't want our help for a variety of reasons, and I agree with them. If they do it themsleves, they then have a better understanding of what it takes to secure and hopefully keep freedom.
The Iraqis couldn't "do it themselves" any more than Russians could in 1934.
The key is to help them while retaining deniability.
The solution is to smuggle in weapons that are identical to current Iranian police and military issue, so that the freedom fighters will have a fighting chance, and their weapons will be assumed to have been "liberated" in street fighting.
The problem is that our limp-wristed DoS has castrated the CIA, so this covert action will probably not happen, and the freedom fighters will be slaughtered.