Possibly... (I can't guarantee that this will work)
You could have the folder action script trigger another script (an applet)
This second applet could have an "on idle" handler - which means it runs repeatedly, with a pause of your choosing between each execution.
The applet script could have a few 'properties', which will persist across the executions.
You could use these properties in the idle handler to check to see if the downloaded file hasn't got any bigger (i.e. the download has finished) - when that happens rename the file and move it to the right place, then get the Applet to quit itself.