Should have been Stock
In my opinion, a vented loop should have been a stock installation. I have a 1985 Catalina 27 std rig with traditional layout. After one particularly bad incident in 1989, I both rebuilt the toilet mechanism (involved disassembly and replacement of all the rubber parts) and added a vented loop. The loop was installed behind the sink and cabinet in the head and at it's highest point, is just below the stbd rubrail. Not a single problem of seawater entry or holding tank backup since. Another answer which is probably more proper/seaworthy is to always close the seacocks when underway (or not aboard the boat), but frankly it is really a pain in the backside to access the valve and it remains open during most times.
