I have been searching for a while now and have come up with 4 options- each a compromise. They are listed in order of highest flow first.
1) 1/2" gas valves. These are a ball valve with a straight through design when fully open measuring .472 ID. They are relatively cheap at $3.50 US each. Downside-Very heavy and since the threaded barb will not screw all the way into the valve there will be some impedance to the flow when fluid flows through the barb, then hits a small ridge as it enters the smallest ID path in the middle of the valve. I have enclosed photos showing hose barbs and a conduit locking ring which will allow you to panel mount these. Another benefit is when you disconnect the tubing you can place an inexpensive screw in plug to keep any fluid from coming out while you are moving the components aroung. It would be interesting to see one of the Dynamos on this forum measure the flow compared to the following.
2) Colder products FL series.
http://www.colder.com/asp_main/flfbind.asp . These are the highest flow, lowest pressure drop, valved couplings offered I have been able to find. These are beautiful, machined aluminum. You can order them with valves in both sides of the coupling. Downsides are weight, $$$$$$$$$!!!, and these are the only couplings of the group that are not manufactured in panel mount. When I spoke with Colder they were willing to manufacture a run of them in panel mount if the quantity were high enough, or a proposal could be submitted to them to add this configuration to their catalog.
3) Colder NSH series.
http://www.colder.com/Downloads/NSHFINAL.pdf . According to Colder these are their highest flow, lowest pressure drop, panel mountable couplings designed for 1/2" ID tubing. Not only are these valved in all sides, but they are designed to only release a few drops when uncoupled. These appear to be the best compromise of the group. You can also order an extra set of opposite sex fittings with no valves, so when you uncouple the tubing, you will still have a valve in the PC side and external WC box sides to prevent a spill. Then you insert the non-valved coupling (with a pre attached section of tubing) into the valved coupling and voila! You will drain the system into a container of your choice, instead of all over your hands, etc.
4) This final item i'm not sure on. At one point in my searching, I found a valved both side coupling with a 5/8" flow THROUGH THE COUPLING! All the other couplings, even though they are available for 1/2" ID tubing, are only a 3/8" flow through the valve body. The drawback to this coupling was it was only available with 3/4" barbs. It was either Colder or Omega.
I guess when it comes to couplings for external WC rigs, it's hard to have your cake and eat it too.