I know this is a little bit late, but if still interested, here is another way to achieve the same thing. This time you won't need a lua script file to empty the text file, but will need a RunCommand plugin measure. So, add the following measure to your code:
Running the above measure, empties the file. To run it, replace the LeftMouseUpAction option of the [Button] meter with the following one: LeftMouseUpAction=[!CommandMeasure "MeasureRun" "Run"].