If the server is running Ubuntu, Fedora, RHEL/CentOS or other relatively recent Linux distribution, the bindfs module for FUSE (https://bindfs.org/) should be in its default repository.
To use bindfs, if for example Syncthing is running as a dedicated user “syncthing” in the group “syncthing” and you want it to be able to access everything under your /appdata/ directory, issue the following command:
bindfs -o ro --perms=777 --force-user=syncthing --force-group=syncthing /appdata/ /mnt/Syncthing
(The user, group, and mount point /mnt/Syncthing can be adjusted for whatever is most suitable for your environment.)
Everything in, and including the mount point, will be virtually owned by syncthing:syncthing with read/write/execute permissions on all directories and files.
And for an optional extra measure of safety, the “-o ro” makes it all read-only – no chance for accidentally deleting any files with a configuration mistake in Syncthing.