(Note: This was tested on Windows XP Home on VMWare Fusion (v2, I think, maybe v3))
I tried the copy/paste trick mentioned earlier, and it seems to work. Loading and saving works fine, and the game doesn't seem to have any issues resulting from the change, like reverting to pre-GOG video crashing issues.
UPDATE: I think I know what the problem is. I checked the directory properties, and the "Read-only" thing being referred to is that the Read-Only box in the folder properties has a blue square in it. This just means it inherits a parent's permissions, not that it is Read-only.
Also, from what I can tell, what actually happens is that when changing directories,
the compatibility settings aren't being forced, since they target C:\Program Files\GOG.com\Riven\Riven.exe, not any other directory. From what I can tell, the game won't save or load properly when in Win 95 or Win 98/Me Compatibility, and in any other compatibility or no compatilibity it won't start at all.
This last part is most curious, because the game did start the first time I moved it from one folder to another, but after a restart, or maybe a change in VMWare settings, stopped working at all. When it did work after changing folders, the game would experience crashes similar to the ones experienced with old versions of Riven, which is probably quicktime related.