My box is connected via Ethernet and I cast my iPhone which is on the same network via WiFi with AirPin: http://www.minixforum.com/threads/latest-airpin-pro-v4-0-9-apk-support-ios-11.5095/
I don't think it's possible (I don't have a Miracast enabled screen, so I can't test it myself). Miracast uses the Wifi adapter, but that's disabled as soon as you plugin an ethernet cable. @Marty Miracast and AirPin are different. AirPin works over TCP/IP so it doesn't matter on which interface your device is connected. Miracast is a direct wireless connection to a TV.