Here's the cutscene working after
>protontricks 283290 quartz
on the command line and
>WINEDLLOVERRIDES="winegstreamer="
in the launch options.
(I don't know why I have to disable winegstreamer.dll after installing quartz.dll, but whatever. All I can say is yes, both are needed. If I only install quartz.dll then the cutscene crashes the game, and if I only disable winegstreamer.dll then the cutscene just doesn't play at all.)
So the game is fully playable like this, but I'm going to undo these tweaks anyway, for the greater good. Apparently a cutscene will get automatically transcoded but only if enough people see those color bars, so I should at least do my part by failing to play the rest of them. I've already seen them before (and they're also on disk in MPEG format so I could just watch them outside of the game if I really wanted to see bad FMV).