Ok, never mind - don't need it anymore.
It turned out that ScummRev 0.5 in fact can read Grim Fandango Demo cutscenes and even output them to AVI. The only problem I had with ScummRev was, that once it output one AVI file, it would not write another one (i.e., it would, but it was entirely filled with black). Anyway, a system reboot helped.
So I am proudly presenting a rare German trailer for Grim Fandango. It was released on a limited Demo/Trailer CD and in some games mags. You can't get it anywhere now.