"If somebody started a company in one of the "estates" areas, and hired the mohammeds, and fired the ones that didn't work out, what labor inspector would dare to show up in a no-go area to make an issue of it?"
This is called the black market.
All European countries have economies considerably larger than the official statistics because the black market provides the goods and services which over-regulation make uneconomical at the margins. Italy probably has the largest economy in Europe, for the black market there is probably fully 100% the size of the legal market.
In France, especially in the banlieux, there is a thriving black market. But, of course, there are limits on what a black market can do for someone. Money cannot be put into the regular, secure banking system or investment system, as it will be detected. Money also cannot be put into registered assets such as real estate. One is at risk for losing anything fixed when one indulges in the black market.
So just how socialist is France? You can't just start a business when and where you want? Are there no private companies?