How to pirate
You wait for one of the miners to go bankrupt before they ever make it to the mining stage, and buy their ships for pennies on the dollar.
Though really, if there are no laws against piracy in space, it wouldn't be illegal, so nothing would stop someone from forming a company called "Space Pirates, Inc." that specifically says it will launch ships to go steal someone else's loot when they try and bring it back from the asteroid belt!
The large majority of the expense of space mining would be in either developing robots that can do the mining unassisted (they can't be remote controlled from Earth due to being dozens of light minutes from Earth) or in sending men out there to do it. Compared to those costs, the cost of simply launching a ship out of Earth's gravity well is a pittance. You don't need to go to the belt to pirate, just hang out at L2 and wait for someone to send their stuff back. Your ship "grabs" theirs and redirects it from its original destination to wherever you want. You can afford to have a larger ship and a lot more fuel since you are close to Earth, and the return mission ship probably taking the 'long slow journey' to save cost, so you can always overpower the rockets on it.