Hello,
in case someone's still looking: I've solved this problem by writing a PowerShell script that sits in the game's folder on a USB stick, always copies the save file into the appropriate place, runs the game, waits until you stop playing and close the game, then copies the new save onto the USB stick.
So basically you just need to start the PowerShell script and it works in Portable mode.
Works well for me so far.
In case anyone else is interested, I could publish it somewhere.
If there is a better solution, pls let me know.
Marginally related:
