Thank you for your lightning fast reply (honestly).
If you take the contents of the saves folder from any machine and put in in any other machines saves folder you will have all your progress.
Already tried that, didn't work. Then, I did the same when finding out where GOG edition puts its savegames: Started a new game, saved, and looked for changes in the directory structure. There is no folder "saves" within GameData/Maps. However, I can still load the savegame, which seemes to be stored in GameData/Maps. The 'Saves'-folder I copied over (again) is still blithely ignored.
A search for the files "WFTO_Progress" and 'WFTO_Tutorial' turned up nothing (after deleting ignored 'saves'-folder again), cpoying them into GameData/Maps did nothing, either.
I'll probably later take a snapshot of the filesystem, bash a few missions, snapshot again and compare diffs. Maybe I'll find something. I'm just a bit baffled, at the moment. Probably need more coffee.
Apart from that, great game, great fun. Been playing this a while now on steam, GOG edition is still a must have though because of the offline copy I can store on my servers and easily and quickly reinstall from there if necessary (bandwidth is still at a premium in some countries), so thanks for that as well.