My last three boats came from the factory without a vented loop between holding tank overboard discharge pump and thru-hull. Surveyor never said anything. It's a closed system except for the holding tank vent, and the pump has a checkvalve, and the thru-hull is normally closed even when the boat is in use -- so no risk of sinking.

On another note -- I think you should add a holding tank vent. A build up of gasses might conceivably cause a lot of pressure with bad results.