Chris,
As you know, you opened an incident on this and I have looked into it for you. As I replied in the incident, this functionality isn't intended and so won't work as you have it set up. A transaction with an External Data property needs a screen set associated and an action which causes the user to be prompted to choose the file for the External Data property. Without a way for the user to choose the file, the property will not get initialized properly.
It's possible there is a way to automate file transfer as you desire using OpenUI entirely but support is not familiar with it. Hopefully someone here on the SCN has some experience. If not, it would be good to contact our consulting services team to see if they can offer assistance for your needs.
Thanks,
Keith Nunn
SAP Product Support