The file specified by fn is opened and parsed as a popt configuration file. This allows programs to use program-specific configuration files.
What’s unclear is whether it can only be used to enable aliases, or if it can also be used as a general-purpose configuration file to replace the command-line interface, and if so, what format it should be in.
As no documentation I could find explains it one way or the other, I resorted to reading the code.
The short answer is no. Popt (1.16) cannot read command line parameters from an rc configuration file.