Yes I use 10mm copper tube in the connections for the tank bypass valve, which is also incorporated into the tank inflow / outflow unions to get it from 3X 10mm nylon to 5/8 barb. These images show the set-up. Red tubes to the tank / Blue tubes from the tank, and the bypass valve.
Two issues I can think of with using copper tube are that it is very inflexible so you would need to be very accurate in the bending & fitting of it if you are to avoid it straining the blocks and damaging the chips / components they are contacting. It will also make assembly / disassembly awkward.
The other thing is the Festo style push fits, rely on a grip that digs into the tube surface as it's pulled out. It wont be able to do this with copper tube so they possibly could come out more easily in an unwanted fashion if you know what I mean.