However, you can use networking file systems such as FTP and Windows networking. Those are a bit primitive under Syllable, but clients are available
PeterWood
Yes file sharing does require the Virtual Box extensions. I use Virtual Box file sharing to run the Red tests under OS X, Windows and Ubuntu. It works well for me.
A few minutes after posting my question, I remembered that REBOL 2 doesn't run on Syllable so I couldn't easily extend my testing to Syllable.