Examining the syntax of the config.pl file seemed to provide the missing link. 
Just in case someone else comes across this question, attached is the 
configuration that is working in the GUI and here are the corresponding entries 
for /etc/backuppc/config.pl to exclude some folders within a share:

$Conf{BackupFilesExclude} = {
  '/home' => [
    '/*/Dropbox',
    '/*/Downloads'
  ]
};

...
$Conf{RsyncShareName} = [
  '/home'
];

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