How to access BTsync files in Pebble with Gadgetbridge? I would like store only some most critical files as encrypted in my Pebble, stored in BTsync. 
Pin code protection of Sync files is not possible in Android 5.x-6.x because files are stored in a file system access to which Sync cannot close (BTsync). 
The storage of critical files in Pebble should be now technically possible with BTsync 2.3 of Android >=5.0 but I am using 6.0 Marshmallow. 
I received answer from the BTsync support


  If yes, then this shall be possible with Sync 2.3 released recently. You just need to save Pebble app files to a Sync's folder either via android's document provider feature or a file manager. This is already done in Sync and now it depends on the Pebble app - whether it'll let share or access its files.


Motivation


some security keys in Pebble 
some critical documents only locally in Pebble (BTsync is working for Bluetooth connection for local network devices)


Pebble and BTsync

The work which they can make is to consider how the development plan of the BTsync App in Pebble assuming that Bluetooth connection exists in BTsync. 
There can be other limitations. The front-end of BTsync Pebble must be limited in selective sync. I think its control center can be in your phone. 

BTsync


  [Y]ou need to save files to a sync share. You can either copy them from Pebble app folder to sync share, or add the Pebble add folder to Sync - however this might be not allowed if pebble app exclusively locks the folder. 
  [T]o sum up: if you want Sync to sync files from Pebble app on your phone, you need to add a folder with these files to Sync.


Development Plan


BTsync Bluetooth support for local network etc between Pebble - Android/PC. So the BTsync app can detect Pebble and sync selectively files there. (here about the development of bluetooth 4.0 in BTsync)
Some Pebble app OR system feature to allow access to Pebble's files, so in that can you can simply copy them back and forth after syncing.


OR copy files back and forth in Pebble after syncing.



Plan


Use Gadgetbridge to make the connection te Pebble. Maybe, it can allow the read of files on the gadget. 
....


Phone: Oneplus 3
OS: Cyanogenmod 13 stable
Pebble: Classic
Connection client to Pebble: Gadgetbridge
Pebble no longer maintained, so the development stops.