I'm glad my question on the sump pump let to this question because I had the same one but didn't really know it. I think you are correct that there are non limber holes between the engine pan and the bilge. When my wash-down hose was leaking, the water ended up in the bilge but when I had an engine coolant overflow due to broken belt, it ended up in the engine pan. Maybe this was designed this way to keep oil and coolant out of the cleaner bilge area. I ended up using the oil change pump to remove the pooled coolant.
I have a 1 inch corrugated hose starting from the top of the keel in the bilge leading to the hand bilge pump. I think I am going to forgo the extra electric bilge pump and just use the hand pump to remove any water that gathers in the bilge. Since I am fixing the leaking wash-down system, I think the bilge should remain dry. If water gathers when I am not on board the sump will carry it off the boat. I don't really like the idea of a check valve on the existing hoses and I don't want to put another hole in the boat.
Maybe what I can do is drill a hole in the sump well close to the base and put a plug in it. When I'm not on board, I could take the plug out. What do ya think?