Originally Posted by FlyingHorse
This is a problem...primary fermentation is pretty violent, and the wort tends to homogenize pretty rapidly once the yeasties start doing their dance. I doubt you'll be able o count on a difference in SG to keep the fermented wort separated from the unfermented.
This would only be an issue with the first pass. After that it would be a slow add and not a "hey, here's 5 whole gallons of unfermented wort, go at it yeasties!" Basically, the system would be introducing 24-36 oz of new wort per day as it's drawn off the secondary.
ETA: Also, reducing mixing would help tighten up the distribution of the residence time, but wouldn't change the average.