That's one possibility, using some kind of ND filter to cover the small CdS windows above the lens. Perhaps a lens hodd might give me another 1/2 stop or so.
Covering the light meter with ND will give you positive exposure compensation (the camera thinks it's darker than it is and gives you more exposure), not negative exposure compensation. You want -1 stop of EC to use ISO800 when the camera thinks it's got ISO400 installed. Sure you could put an ND on the lens but then why are you using fast film in the first place!?
Sorry, I don't know the camera and don't have a solution for the OP.