Life charge status
 

Shouldn't it be possible, with a very, very accurate multimeter, to tell the remaining charge on a life receiver pack by measuring the no load voltage?

No.
Actually, the no load voltage is not a reliable indicator for any kind of battery.

No lots of airplanes have been lost doing that.

No. Will not tell you what you want. With LiFe technology (or Lipo, or just about any battery), you need to know two things....How many milliamps (watts) are IN the battery when you take it off the charger? and How many milliamps (watts) do all of your servos and rx typically use? Then you do a little math.
NEVER use more than 80% of a total capacity, so to allow a safety margin, use 75%....so, to make the math easy, assume you have a PERFECT 1000 mah battery when it comes off the charger. That means you can TAKE OUT 750 milliamps before needing to charge. Safely.
Now, find out what all your servos use when cycling them all by measuring current being drawn on the OUTPUT lead of the battery. Let's say it's "200 milliamps". To give a safety margin, you'll call that "250 milliamps".

So, you have a FRESH battery that will safely give you 750milliamps for one hour.
Divide that by the 250milliamps, and you have the amount of flying time (SAFE) in HOURS.

You may blow this off as too much work, but it is work that saves airplanes and batteries EVERY DAY.
KNOW your batteries.
KNOW your airborne setups.
DO the math.