I don't think rsync will ever be added to the DNS-323. Simply because it's too complicated: server side (with configuration files), client side, mirroring (backup), incremental backup (snapshot).
You just look at the man page for it and tell me what should be included and what should be left out. D-Link will need to provide a front-end that somehow makes sense, which will cover a sub-set of features. It's a nightmare to manage user expectations.