Android itself can accept proxy settings. You need to go to the wifi connection, then to advanced settings, and specify the proxy details there.
Xibo for Android will then use those proxy settings to pull in external content.
If your using an Ethernet connection, most device manufacturers don’t include proxy support on those devices, so you’ll need to root the device and use one of the Android proxy apps in the market to set a system-wide proxy - which again Xibo for Android will then use.
It’s not possible to have Xibo for Android proxy external content via the CMS - with the exception of RSS feeds. RSS feeds are downloaded by the CMS, processed, and then sent to the Player.