I didn't know there was such a thing as demo firmware.
Well, if you can't get access to the internal storage via USB, you're in pretty much the same situation as people with bricked readers. The device has no recovery mode for SD card; it reads updates and executes scripts on the internal memory only so without access to that you are pretty much stuck.
As far as I'm aware no one so far managed to find an unbricking method.
We have one user here https://www.mobileread.com/forums/sho...d.php?t=181584
who received a device back from iriver and identified some points which they soldered, which may be a serial console; I tried to connect to those but couldn't get any valid output myself.
If you find a way to access the internal storage (other than the regular USB) let me know