I've never seen or heard of a cello riser, but in theory I can see how the endpin could transmit vibrations to the surface of the riser which in turn *might* set up vibrations in the riser. Standing with all of your weight on the surface that you're looking to vibrate, however, is bound to damp/mute a lot of the resultant vibration, so maybe the design should be ammended to include an additional partial platform only connected at the edges to stand upon?
Something that might maximise the volume gain would be design with a lightweight circular plate suspended so it is free to vibrate, with a cup-like depression in the centre for the endpin. I could imagine this might have a similar effect to the resonator in a dobro.