The primary reason for the 30G barrel tied into the water loop is for fast water changes. I can close two valves on the inlet/outlet lines going to the barrel, flip a sump switch, and then water is evacuated from the barrel. I can then flip another switch and pump RODI clean water into the barrel. Add my ferts and traces. Then run the filter and heater for a bit. Once the water is prepared and ready, I can open the valves and turn on the high power pump again. This prevents any chemical/temperature changes during WC and makes a 30G WC done in less than 10 minutes. With my busy schedule, a complicated high tech system that makes changes quick and easy is a major relief.
Ok.. gotcha'. I must have misinterpreted what you were saying. I assumed you were more concerned with having more volume.
Actually pulling it all off is a cool idea. Nothing wrong with making water changes easier.