ssh is what I'm really after.
opkg update && opkg install dropbear-ssh
Dropbear is a small SSH client/server implementation suitable for using on embedded systems. It's largely compatible with OpenSSH and mostly works the same way (e.g. you can create a /mod/.ssh/authorized_keys file to enable key-based logins).
This package uses passwd/shadow files installed in /mod/etc/dropbear/. The only user that's defined when you first install the package is root with a password of humax but you can change the password or add new users by hand and subsequent package upgrades won't overwrite it. That directory is also where the server keys are placed - they are generated the first time you install the package.
SCP's supported but SFTP isn't just yet. SFTP will be a separate package coming shortly.