Probably yes, so just try it! If any user-configurable env var is missing, I'll add it right away. PhotoPrism already has a LOT of configurable env vars and I don't recall seeing something about S3 there. So it may be inside the app.
If it’s not from within the app, then I’m afraid it’s not possible with the current setup. Also not sure if this would be stable enough. I believe PhotoPrism allows mounting webdav from within the app and import pictures from there.
