Well many Android apps only work with media that the internal Android media database knows about. I believe the media data scanner does all sort of very heavy and intense file operations such as extracting cover art, image frames and metadata from media files. So beyond a small trivial number of files this won't really scale.
I found one OSS app- Simple Gallery Pro in the F-Droid store that allows me to manually set a path. I set it to a test folder with 3 image files. Unfortunately 30 minutes later it still could not display the files. I am monitoring the rclone log in real time and I can see it chunking away but I believe it was trying to read the whole file and create a thumbnail, again a very resource intensive operation.
As an alternative, I thought I could try to use a local copy of the data, point the scanner to this folder than change the paths from the local files to the rclone version. It would depend on the database configuration however.
I was wondering if anyone else has tried to do something like this.
Technically it should be possible to use rclone in this direct way. Google photos does something like this but there is no OSS alternative Rclone to me seems to be a "holy grail" like file application project and I am searching for it killer app. I haven't really found anything that really works beyond sync backups yet.
I would love to hear how others are using rclone for any application in extraordinary ways.