Because root's home is in a tmpfs partition, I patched dropbear to always look for the pubkeys in a single place: usbnet/etc/authorized_keys (and tweaked the permission checks to not die horribly because it's a vfat partition).
It handles files formatted for OpenSSH, so you shouldn't have any trouble with a key generated by openssh's tools (be it a rsa or dsa key)
. (It works fine with openssh's ssh-agent too).