You were right : I managed to read the movies from the original .sit archive. I put that .sit archive, with the "ressource fork", on my site : http://jonathan.lafarge.free.fr/downloads/FILMS.SIT You can extract it with Stuffit.
However, the only way I found to read them is to read them from a Macintosh Emulator (available here (70MB, 500MB deflated) : http://jonathan.lafarge.free.fr/downloads/BasiliskII.zip
It's not really convenient, as the video and the sound are not synchronized (the video goes too fast).
Does someone know how to convert the video files in the FILMS.SIT archive above into a standard Windows video file format, readable on any PC (like .avi, or DivX, or else) ?