I do not remember someone having done exactly the same thing.
You can do it yourself however, thanks to this trick:
https://www.mobileread.com/forums/sh...d.php?t=321609
You would need to write a script which downloads an image from the given url every X minutes and place it in the ".kobo/screensaver".
You can launch the script with Nickel Menu. Or, if manual updating is enough for you, I believe a one-line code started by nickel menu's cmd_spawn or cmd_output could be enough.
I do not even know the basis of bash so I won't be the one helping you, sorry!