'OK Google' now works with the S View cover closed. Don't know when that changed but it didn't used to work, just went 'plink' and then nothing unless you opened it first.
I turned hotword detection off a long time ago to save battery. Found I never needed the feature. Only time I did was in the car but I have a steering wheel button to trigger voice commands so it's unnecessary to have voice detection enabled.
Just found a problem though. If I set a timer with the cover closed it doesn't actually start counting until the cover is opened.
I guess someone will notice and fix it sometime.