I was just trying to do the same tonight, but without hope.
Trying to call rsync resulted in this error:
sh: rsync: not found
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: remote command not found (code 127) at /SourceCache/rsync/rsync-42/rsync/io.c(452) [sender=2.6.9]
Trying with sftp, still no success:
sh: /usr/libexec/sftp-server: not found
I've read about support of optware, but the readme.txt on that site is gone. I've read that the new firmware support a plugin architecture, but noone is talking about an rsync module, so I suppose it's not there.
We'd better ask the guy who made the SSH plugin, AFAIK the only "unofficial" plugin for the MD-253.
I tried to create the toolchain for the MD253, but gave up, being such a job a too dark path to follow
in almost no spare time.
Meanwhile, I'll resort to ftp sync... I wish I never bought this faulty box.