So you finally got it finished then Roger? You're right, it was probably around 2 years ago that you dropped this idea into my build thread
I didn't respond at the time because I couldn't decide if it was a good or bad idea.
To jump on the bandwagon a bit 1st, forward facing snorkels don't tend to have any problems with water ingress in heavy rain and the induction system can cope with quite a bit of water because it is suspended in a large volume of air. Rear facing is thought to be better for fast road use in dusty conditions but in reality the dust is suspended in the air being drawn in whichever way the snorkel faces. The whole question of the best way to draw air in is covered in an excellent book "Race & Rally Car Source Book", it's not as simple as most of us think!!
In response to earlier points raised:
You should try to have the windows open before going into deep water, it's difficult to open a door when it's that deep, so air will be available for the engine and driver.
Induction noise is a small price to pay to ensure the engine keeps running.
If the valve has proved itself in testing then there's no difference regarding risk than having a snorkel. For example, once you've fitted the snorkel and tested it's watertightness, has it subsequently sprung a leak? You'll only know once you've ingested water
The dirt on the back of 4x4s is created by the updraft created by the car passing over the dust not by a downdraft from the roof. Simple to stop it happening by fitting a small roof spoiler as seen on lots of road biased 4x4s. They direct the air downwards from the roof to counteract the updraft.
I'd expect Roger to have designed the valve so the direction of airflow will assist in keeping the flap closed?
A nicely presented 4x4 is always less offensive to walkers than a rough looking one and especially a well and considerately driven one tends not to get them waving sticks
Over-riding all of the discussion though, don't go into deep water unless there isn't an alternative. It's not just dangerous to your engine