Hello Frank
I am currently working on a new post of a Submacro and a Subroutine checking for things like a held fn key but this will not be ready for you in time.
In the time you have to wait you can of course try something your self borrowing the ASObjC code posted in the OP of my Multi-Press Templates Post.
If I remember it correctly it has a routine for watching the fn key built in so you can use it as it is for your purpose.
If you have any questions about that piece of code just ask me
Greetings from the sunny city Wetzlar in Hessen 
Tobias