As you've found, liquid water won't let the joint get above the boiling point which is not hot enough to melt solder. They make some plugs that dissolve that will give you awhile to solder the connection. In a pinch, people also use white sandwich bread (no crusts, no fancy stuff with seads), which will also disolve when flow is restored. It can leave bits around, but does work. There is also a tool called Jet-Sweat you may be able to rent (they're expensive to buy), and there are machines that will freeze the pipe to make a plug. Open a valve higher up to break the vacuum (think finger over a straw), and wait longer for it to drain off.
Keep in mind that you must have an opening for the steam and super heated air to escape on the 'last' fitting, or you'll get a pinhole leak, causing the fitting to leak. You can do this by leaving a valve open in the line (preferably close).