This is my problem with the rudder / water pickups that I have now.  I need to tune my boat, so I am trying to figure out how to get water flowing through the head and so forth, but have the boat stationary.  This is why I was thinking about installing the extra prop wash water pickup.  Since the prop will be spinning, it should force water up and through the new water pickup if it is at 2 - o - clock behind the prop, even at idle.  

My current problem is that the rudders are obviously on the sides of the prop so, unless the actual boat is moving through the water the in-rudder pickups don't work. 

I thought about rigging my own system to force water through the head using a good size aquarium pump.  I have experience in working with aquarium pumps as I have a 75g reef, so what I was thinking of doing was to get a good sized pump, put it in a 5g bucket of water then take the water lines that go from the top of the rudders to the back of the transom off and get two new longer pieces of water line.  Connect them directly to the back of the transom and shove the other ends of the longer water lines in to the nozzle of the pump in the bucket.  Turn the pump on and it should force water up and through the head and the exhaust flange.  If I need to keep adding water with the hose then I can, but my family has an actual jet boat and this is a similar way we use to cool the big motor when we are tuning up our actual boat.

Except we don't have a pump, we just use the hose water pressure, obviously.

When I was at the races I saw a sick looking external pickup that was similar to what you are saying RON, it mounted to the hull and then was basically a bent aluminum tube that came down right behind the prop, then had a through-transom hose fitting on the other side, it was really sweet.  I can't find it anywhere though.