This is possible
Have now rejigged the arangement:
G5 is Card Reader and Monitor which has Keyboard/Mouse Connected - This all works fine.
On the USB Card I now have 2 x printers neither of which are turned on at startup and my iPod dock which has no iPod connected and also my Canon Scanner which in theory is now the only device needing power.
I would say however that at least half the time my machine won't recognise the Scanner on startup - maybe it's just a dodgy scanner over USB. Might have to buy another...