I have since added a 480x320 touch screen to my setup. When powered by a 10000 mah battery band it lasts over 12 hours, even with the screen always on. So it should suffice for my longest typical event shoot of 12 hours.
The screen is handy to quickly see if the pictures are being uploaded. I also allows you to start the capture program without VNC. My original plan was to mount the raspberry in a grip but until I find a suitable grip, this is not so bad!