Hey,
as you can push a message from the smartphone app to the garmin edge 810 display: "14% of battery left", you could implement a message function on the live tracking site sending to users who watch the rider/runner. I think of a simple message box, where you can place few words like "go go go ... faster dude!" and it will be transfered through the smartphone app directly as a flash message onto the edge device.
This would be cool and there could be some interaction between the watching and the riding dude ;-)
cu on the road