Sure, you can. My Android phone has an option in the Storage section of Settings where you define the default destination for writing (storing your files). I just checked SD and all the GPS tracks and pictures are stored there.
Sadly, Open Camera also doesn't allows me to turn off the screen either. I don't really see the advantage of Open Camera over the default Mapillary app.
Yep, Pokemons have come to the rescue for Mapillary people!
Today I was on bike. I stopped to let some joggers pass and putted down my smartphone. I still have a good hearing, and I heard one jogger say : "That's another half cooked one".