Even if your method were possible, which it isn't as described since it would require both rockbox and the pc to have access to the filesystem, there are far better and simpler ways. If the pc is able to write the file, rockbox cannot read it until disconnected. If Rockbox can read it, the pc cannot write it. Unless you're seriously wanting to connect and disconect rapidly, this can't work. And even then it wouldn't be able to control responsively or respond to hold-then-release actions well. There are simply much better ways than even an ideal file-based communication method.