cool thanks for your thoughts guys - are there similar tools for mac?
Richard's method could be made to work for Mac (Exiftool runs on Mac too), but his recipe would need modification. I think you'd have to write a shell script that invokes Exiftool with the appropriate command-line arguments, and then have the LR export post-processing action invoke the shell script.
The two plugins I mentioned do that for you automatically, and they run on Mac.
