For the $wmgMonologChannels setting, I want to add three new log settings for the beta cluster that are unique from the values that are set in $wmgMonologChannels['default'] for production hosts:

// Additional log channels for beta cluster
'wmgMonologChannels' => array(                                          
    'default' => array(
        'CentralAuthVerbose' => 'debug',
        'dnsblacklist' => 'debug',
        'squid' => 'debug',
    ),
),

Unfortunately, the wmfLabsOverrideSettings() function that merges the values provided by wmfLabsSettings() only handles the merge at the top key level (eg wmgMonologChannels). If the top level key is prefixed with a - then the beta settings replace the production settings. Otherwise the production settings and the beta specific settings are combined using array_merge:

foreach ( $betaSettings as $key => $value ) {
        if ( substr( $key, 0, 1 ) == '-' ) {
                // Settings prefixed with - are completely overriden
                $wgConf->settings[substr( $key, 1 )] = $value;
        } elseif ( isset( $wgConf->settings[$key] ) ) {
                $wgConf->settings[$key] = array_merge( $wgConf->settings[$key], $value );
        } else {
                $wgConf->settings[$key] = $value;
        }
}

The problem here is that array_merge is not recursive (by design). This leads to the $wmgMonologChannels['default'] entry from the beta settings overriding the $wmgMonologChannels['default'] entry from the production settings rather than my desired behavior of merging the two $wmgMonologChannels['default'] collections.