Thanks for the nice recognition, Onemoreproject; you're more than welcome.
In the case Nigel Calder's diagram covers, the flow in that part of the circuit is under pressure when the engine (and hence salt water
pump) is running.
I forgot to return to the situation you raised:
<<I am wondering if I could install a vented loop on the centerline of the boat that is at waterline level and run a hose from the hole in the loop up to deck to elevate the siphon break well above the angle of greatest heal.>>
In your case you, too, have a "dynamic" pressure situation, because the bilge pump, forcing flow through a length of piping, will be fighting the resistance to flow in that pipe.
And that resistance to flow will also force the water up your vent hose.
The easiest way to measure the equivalent "static" pressure is simply to use clear hose for the vent, and empty a bucket of water into your bilge sump.
The distance the water climbs up the hose tells you the "static head", and it will always be above the waterline (provided either that you do this test with the boat in the water, or the outlet fitting through the hull is above the waterline)
You need to add a margin for heeling/pitching if the open end of this hose is not central, plus a margin for 'heave', plus a margin for the hose or outlet blocking up somewhat with marine
growth etc. (which will increase the 'back pressure')
And, if it's easy to do, an added margin in case at some future time you upgrade the bilge pump to a higher throughput, and forget to reestablish the necessary height of the vent for the loop.