Copyright © 2003 – 2008 Michal Čihař
After spending few hours (not intensive work, I was doing also other stuff) with duplicty and dropbear, I finally got the idea to enter magic words to Google to find solution.
Those words were excitingly "openwrt sftp" where first link points to sftp with Dropbear howto.
Next time I should start with looking for documentation, most likely somebody already had same issue…