Hi Andreas,
This is an excellent question and something that I can see is missing from our documentation.  
You will need to implement the "IFinalizeLogEntry" interface.  Your implementation will be something like:
/// <summary>
/// Implementing IFinalizeLogEntry will give us a chance to process any unparsed data.  Unparsed data will
/// automatically be added to the OriginalLogEntry, but you may want to process it further.  Unparsed data
/// will be a collection of all lines where "IsLogEntry" returned false.
///
/// Here, we are adding the unprocessed data to the message for display in the LogViewPlus grid view.
/// </summary>
/// <param name="entry">The log entry to be finalized.</param>
/// <param name="unparsedData">Any data that could not be parsed.</param>
/// <param name="result">The result of the log parse.</param>
public void FinalizeLogEntry(LogEntry entry, string unparsedData, ParseResult result)
{
  if (string.IsNullOrEmpty(unparsedData))
    return;
  entry.Message += unparsedData;
}
I will put this in the sample code and update the documentation.  Thanks for bringing this to our attention!
Toby