It doesn't work, of course.
The bigger problem: if data is continually written to the drive by other programs at the same time, then your save file is wiped away completely at the moment you try to save the game again on an existing slot. I assume this happens because GZDoom deletes the old save first before writing the new one, but if the space runs out during this process the new file can't be created - or is corrupted, resulting in a 'Can't Read Data' message when you try to load that save. Basically, the save file is empty. 0 KB.
I noticed this yesterday during a round of TNT, and realized too late what was happening, and that I should've kept the window open and the game session alive till I cleared some space. Instead, I thought that shutting down the game and opening it again might fix the error... and that was my error. The current game was gone, the save was empty, and apparently so were my settings.
I hadn't changed the settings, so I'm not sure when this happened, but I assume that GZDoom attempted to save the IWAD selection when you open the game upon my second launch (when I realized the save still wasn't alright, and looked around to see what else could be the issue).
So that's that. Morals of the story: keep plenty of hard drive space free! Backup your data! And most importantly: save to a different slot occasionally if something goes wrong with the current one. Good practice also in case you save at the unexpectedly inopportune moments, like when an invisible demon you somehow hadn't noticed has just crept up on you and is about to bite you in the back.
Oh well. I did follow the latter two of those rules, though the last one a bit too infrequently. Time to run through a few levels again to get where I was when I left off last, and maybe clear up some space before I do...