It likely won't be possible to persist files across browser reloads if using the site as a guest. However, with the new "buckets" feature for file sharing, perhaps there could be an option to convert your guest session into a new bucket.

It seems that a shared project basically creates this functionality if you know the URL. Not that I'm completely happy about that!

Indeed, sign in only makes sense when there is need to save the file in the cloud, and not just for creating/editting.