Unfortunately, substitution is done on reference not at parse time.  IIRC, the cross-section limitation is simply part of python's ``configparser`` module: https://docs.python.org/release/2.6.8/library/configparser.html?highlight=configparser#ConfigParser.ConfigParser

There is an option to extend substitution with a custom ``optionxform`` method, but these things are beyond scope of our charter at the moment.  Really I'm mainly most concerned about getting a wide variety of (simple) tests in and working.  The more efficiently we achieve our content goals, the more time is left over for playing with the "interesting" parts :)