Just to add, account of some possible naked eye observations of Callisto and / or Ganymede. Rarely, seems you get reports of seeing them even without Jupiter obscured. Though have to watch out as sometimes Jupiter passes close by to a star which can look like a moon to naked eye.

Denis Dutton on naked-eye observations of the moons of Jupiter

Bitesize Tour of the Solar System

And more here about seeing the moons with naked eye, suggests a small amount of moonlight is actually beneficial to help reduce the glare of Jupiter.

2007 The Nature of the Dawn's Heart Star Page 55

This diagram from the paper shows that a normal observer would not be able to see them under any conditions but a keen eyed observer in ideal conditions could potentially see all four. A small amount of moonlight or twilight might help by diminishing the glare of Jupiter.

That's from a paper that says that the Bushmen may have seen them as an alternative explanation for one of their legends. Their dry desert conditions would have been ideal. The Nature of the Dawn's Heart Star Page 54