[GpsFrontier]

At night, after the sun goes down and the stem, branches and leaves cool, does the flow through the plant vessels actually reverse ?

Again I am not a botanist, but I would say no. It's true that as the leaves absorb the sun, they do evaporate the water that is inside them (stems and flowers also). This evaporation causes a siphoning action through the capillaries that suck up moisture from the soil. But to the best of my knowledge the moisture in the plants don't drain back down into the soil when it becomes dark (night). If they did, they would wilt dew to lack of moisture just the same as they do when not getting enough water during the daytime. And if they are wilting even at night, something is wrong.

The exudate process is more of a survival instinct the plants have developed over the years. Much like glands in a human. Like sweet glands to cool the body when it is hot, or saliva glands are used to help chew and digest food.

In the case of plants they have all kinds of survival techniques from a particular color or smell, to small hairs that tiger a flower to close like in a venus fly trap. In the case of root systems producing and discharging any chemicals, I'm sure the exact function of discharging it is just as plant specific as the function itself is to the particular plant. Also how often to dispense it, as well as how much it needs is all plant specific. I am sure that also probably depends on the conditions as well. In other words the plants would be reacting to the condition in witch the chemicals were designed for, and discharge them accordingly. Like a snakebite, a snake does not bite everything in site. it only bites food or a threat, even then it determines when to dispense venom and how much to inject.

I'm thinking along the lines of totally dispensing with the nutrient solution at regular intervals, perhaps daily.

Not sure exactly what the concern is. Changing the nutrient solution regularly is common, everyone has there own recommendations on when to do this, But I have never herd of anyone doing it daily. If going that route I would suggest using a non recovery system. In witch case the nutrients would not be recirculated back into the reservoir. You would just add more fresh nutrient solution to the reservoir as needed (how often would depend on your setup needs), and what was used would simply just run off.