I have one
I have also seen photos on advRider where someone bought the electrical mounts, but then bought one of the small Pelican cases to put electronic equipment in instead of using the tank bag
When you buy the Motech stuff:
The ring for the tank comes with a wiring loom - that also contains a magnetic switch and a relay so that the tank ring is not live when the upper ring that goes on the bag is not fitted.
The ring for the bag contains a pull bar for release (which is magnetic to trip the relay) and some trailing wires (terminated in automotive banana plugs) that you feed though a hole into the bag.
You have to drill the bag to secure the ring - and pass the wires through (5 holes) As such you could easily attach any smallish container instead of the official bag.
The bag itself has some optional extras available - i.e. a map pocket and a wiring adaptor the takes the banana plugs to a car style lighter plug and a BMW style Powerlet plug.
For some reason although both the map pocket and one of the side pockets on the bag have rubber cross hair cut outs to allow cables (i.e. iPod headphone leads) to get out - the main compartment does not - although you could easily drill another hole in the underside or just pass a wire through the end of the zip.
The bag itself expands and comes with a waterproof cover that I have used at motorway speed in heavy (but not torrential) rain.