Offline: an app can't be offline and collaborative in the same time. However Ziteboard works well if no internet connection. Collaborators can draw offline and all drawings will be synced after reconnection.
Self-host: Ziteboard doesn't offer on-premise solution, but something close: true white label solution. The app can be customized to your brand and needs. The service runs under your domain and can be embedded into your page, web app.