On my last boat I installed a Lavac with a "drop through" holding tank. This system is common on European sailboats especially in charter.
The system worked extremely well. I would only change two elements.
1. I would surface mount the Blake pump to make it easier to service.
2. I would take the hull pumpout from the top of the tank and use a macerator pump to empty the tank at sea. I would loose the "drop through" element but I did not like having fittings on the bottom of the tank.
The Lavac is a great heads. Check it out here