ballast question

My system will have a scoop thru-hull and a brass ball valve before a homemade manifold. I would like to avoid the RV waste valve, but at the same time, I won't be able to reach all the way into the bilge to shut off the ball valve each time out. My question is, can I have plastic ball valves (in convenient locations to switch off) on each inlet hose before the sac to block the incoming water? Will water pressure be very high this far up the line where I would need to worry?

I know some people have talked about their sacs filling while underway with scoopers, even with the pumps off. I guess I'm wondering if the water pressure is very high when this happens?