Are there more instances, in the Bible, of people turning into pillars of salt (Gen. 19:26) and does it have a specific meaning?
There are no other instances in the Bible of becoming turned into a pillar of salt. Regarding the meaning of salt there does not seem to be a symbolic correlation of 'salt' with the judgment that lot's wife suffered. In fact salt generally means to preserve, like an antibiotic due to its pure composition and ability for preserving meat from rotting. I suppose if we said she was condemned absolutely or 'purely' we might extend the metaphor but that seems forced.
The fact that she became a solid rock-salt-like figure who must have stood there for sometime does have meaning. She was turned into a memorial of death for all to witness. Even today the region where Sodom and Gomorra was said to be (around the river Jordan and the Dead Sea) is filled with unusual salt levels. Swimmers can even float in the water. This long lasting memorial shows that loving the world and turning our eyes to gaze upon it when we should be fleeing from the 'wrath to come' by turning to Christ through faith, indicates that all unbelieving souls will ultimately be judged in an absolute and severe sense.
Maybe the solidity itself has meaning too, beyond its lasting memorial effect. The heart that is wicked is often referred to as 'hard' in the Bible. Hard hearted is a sign of rebellion and wickedness. In this sense not only is she a memorial but it shows that loving the world makes one absolutely hard an insensitive to God and his love. But this is again possibly a slightly forced metaphor. The striking point about the story is its simplicity and avoidance of any further explanation. Maybe that is more to the point. God will judge the sinner and there is nothing complicated about it. No alternate solutions will be found. It will just happen while standing dead in one's tracks.