Where are reading positions saved in the filesystem?

They are saved in Android system maintained SQLite database, which can be accessed only by the app itself. Android permissions system does not let other apps or scripts access it, unless you would root your device and run under root account...
 
Maybe you could use @Voice Bookmarks feature instead, but you would need to bookmark current position first (long-press a word in text, when the "reference toolbar" appears, press the bookmark icon). You could then use the "Export selected" function on the menu of @Voice Bookmarks screen, or read the .bmk files in .config directory under @Voice default folder location. To find what is that default folder, open the Settings screen in @Voice and press "@Voice folder location", look at the top of screen.
 
Greg

Also thank you for this info