Thanks for the details, Mr Blight!
So, if I understand that correctly: If a drafted CHLer is not signed in two years they either A: reenter the draft if age < 20 or B: become a UFA if age > or = 20.
If a player still does not sign within two years of the second draft they become a UFA.
So UFA eligability after the 2nd draft would happen at the same age as a player leaving college (if they stay the full 4 years). So it's not really a unique loophole to NCAA players, anyone can do it.