Folks,
I just figured out a nice way how to use sftp backend against Synology NAS with checksums without grand administation access to the used user.
So, in rclone.conf is similar:
[nas]
type = sftp
host = ...
port = ...
user = ...
pass = ...
shell_type = unix
md5sum_command = md5sum
sha1sum_command = sha1sum
path_override = @/volume1
and now the tricky part.
You must change /sbin/nologin to /bin/sh for used user in /etc/passwd on Synology.
I've tried to use a custom script which limits scope of executed commands to md5sum and sha1sum... but seems that here a fallback to /sbin/nologin.