Hopefully that is a possibility in the future. The OP did mention the camera's HDMI and AV outs (presumably to feed into the Surface?). As far as I know the Surface can't accept HDMI input and it doesn't have AV input so that isn't an option. I suppose a USB cable connection is possible using an AV port or HDMI usb dongle of some sort.
Otherwise I think to do that you would need a direct USB connection to the camera and for an app to facilitate the communication as you said.