I found only one device on the same StorLink chip. Thanks John Allen for info about ICY BOX NAS1000.
ICY BOX NAS1000/2000
Alternative firmware: http://nas-2000.org
This device is builded on the same SL3316 chip as DNS-300. Also it has sources of firmware here: http://www.raidsonic.de/en/pages/service/driver.php Seems to be it uses open source HTTP server instead of proprietary in DNS-300, so maybe it's possible to extend it.
A little bit info only: http://reishiki.dip.jp/blog/2006/08/03/000159.html
NOTE: It has kernel 2.6 and no open sources yet.