In our deployment script we automate the installation and configuration of our product. For Development and Test environment we want to log everything to a file. The powershell script below takes care of adding TextWriterTraceListener to the trace listeners.

$configFile = "YOUR_CONFIG_FILE"
$xml=New-Object XML
$xml.Load($configFile)
# locate the sharedlistener with name TextFileLogListener
$tracelistenernode = $xml.SelectSingleNode("/configuration/system.diagnostics/sharedListeners/add[@name='TextFileLogListener']")
if (!$tracelistenernode)
{
    # filter on eventtype Information
    $filter = $xml.CreateElement("filter")
    $filter.SetAttribute("type", "System.Diagnostics.EventTypeFilter")
    $filter.SetAttribute("initializeData", "Information")
    # TextWriterTraceListener in shared listeners
    $tracelistenernode = $xml.CreateElement("add")
    $tracelistenernode.SetAttribute("name", "TextFileLogListener")
    $tracelistenernode.SetAttribute("type", "System.Diagnostics.TextWriterTraceListener")
    $tracelistenernode.SetAttribute("initializeData", "c:\\log.txt")
    $tracelistenernode.AppendChild($filter)
    $xml.configuration.'system.diagnostics'.sharedListeners.AppendChild($tracelistenernode)
    # TextFileLogListener in tracelistenerd
    $listener = $xml.CreateElement("add")
    $listener.SetAttribute("name", "TextFileLogListener")
    $xml.configuration.'system.diagnostics'.trace.listeners.AppendChild($listener)
    # write the config changes to disk
    $xml.Save($configFile)
}

The filter is set to Information. This means everything is logged and you can end up with a huge file. Feel free to use the script, but you do this on your own risk.