Is ssh the only way? Is there a way to create a root privileged ftp user? Does Vsftpd support that? And if so how would you do it? So far, the best alternative I can come up with is lxde and xrdp but I don't want to slow down my juju nodes like that.
Once you deploy the instances they are normal Ubuntu Server machines, so you can admin them any way you want to edit files.
Though keep in mind that many public providers do not open ports by default, so to get something like xrdp working you'd have to configure that afterwards.