Thank you for the suggestion! Unfortunately I think that a feature to clean the export folder is a dangerous idea. I can imagine many people will turn it on without understanding what it does, and then lose important files.
I also think that an option for running scripts before the export is too specific – the only use case would be to clean up old files in the export destination. If this is what you need to do, you can do something like richards suggested, and delete files that are older than a couple of minutes in your regular export script.