Ok, I can confirm that putting the tv_grab_file.xmltv file in the ~/.xmltv/ folder of the user account that I'm accessing tvheadend from does not work. I get the following output on tvheadend's System Log window:
Mar 09 11:09:35 xmltv: Grabbing "tv_grag_file is a simple grabber that just read the ~/.xmltv/tv_grab_file.xmltv file" using command "/usr/bin/tv_grab_file"
Mar 09 11:09:35 xmltv: No output from "/usr/bin/tv_grab_file"
I also tried creating a .xmltv/ folder in my /root directory and creating the file there, as root. I still get the same System Log output.
So where do I put it?