Evan McElravy writes to say that I erred; the cheap buses were not mentioned on his blog. But it seems that the Chinatown lines aren't the only instance of this phenomenon. Writes McElravy, "There is a nightly (except Fridays of course) bus service between Montreal and New York, run by some orthodox Jews. It advertises only on synagogue bulletin boards and the like, but does a good business. I'm told the whole thing is absolutely sketchy in every detail, and speculation is rampant about what must be in the luggage hold."
So there are plenty of alternatives to Greyhound, you just need to know where to look for them.