Hi gnhb, thanks for adding it. I tried it and it works fine, but there is one problem.
If I click save on Inerfaces -> WAN page empty service name is not added to mpd_wan.conf, so I have to go to PPP config page and click save, only then I am able to establish PPPoE connection. Also if I save config on PPP page, then save on WAN page and then go back to PPP page 'Configure a NULL Service name' will no longer be checked. I think it would make most sense to have 'Configure a NULL Service name' check box available on WAN page (if PPPoE is selected) as well as PPP page so it's less confusing to set it up.