The man page ld.so has ENVIRONMENT section that looks pretty good.
Otherwise one of the most reliable options is to simply grep the source code for calls to getenv, I would not be that surprised if their are some extra ones that are undocumented.
Used this once to debug an issue where it turned out unwanted exports in one library A where used to satisfy imports in another library B and thus preventing library A from being unloaded. Fun times.
The ldd command is also essentially just a wrapper script that sets LD_* variables that get the dynamic loader to do all the real work.
