What is an example of an extproc.ora file?
Why do you expect that "there must be more to it"? extproc.ora simply sets environment variables for the extproc daemon, which is used to execute code for external stored procedures, and for procedures themselves. There is some documentation in the application development guide. 

The one environment variable that seems to be recognized by extproc itself is EXTPROC_DLLS that you mention, which determines the locations where the daemon looks for shared libraries. If set to ONLY:&lt;path&gt; it won't look in $ORACLE_HOME/bin or $ORACLE_HOME/lib. 

Any other variables set in extproc.ora are simply passed on to the environment where the external code runs, so they would be specific to each library.