I use ejabberd as chat server and as interface to hubot. Solves both authentication and backlog issues. With a ssh tunnel (or your choice of vpn), it also solve

I use ejabberd as chat server and as interface to hubot. Solves both authentication and backlog issues. With a ssh tunnel (or your choice of vpn), it also solves out of network access. Jabber and ssh clients exist for Android (and iOS I presume)