Use the lower radiator drain plug (NPT threaded) for a quick & easy 'return' access. Where you pull the feed from is up to you. Lots of option, but I like the block drain plugs. Also, water cooled turbos don't really 'flow' water though the turbo housing in the way the engine is forcibly cooled. Water cooled turbos just use convection of the heated coolant to move coolant though the system....aka hot water will naturally flow out of the turbo drawing cooler water in. It isn't really pumped through. Not sure if your objective actually falls inline with this concept.
"Aspice, officio fungeris sine spe honoris amplioris." (Face it, you're stuck in a dead end job.)