That sounds good but maybe not the best thing... Think of it this way. if you put a plant in a acrylic box and measure the amount of CO2 in it then seal the box and wait till all the CO2 is gone the plant won't be happy... then you determined the rate at witch it took CO2 out of the air and got the air to be pumped into the box at that rate your plant would have huge die off and not be able to remove as much as otherwise... on the other side if you were to give it elevated levels of CO2 it would take off and start to convert more CO2 then before.
Now with your experiment the plant will not die because you are providing CO2 but the nitrate/phosphate is what it uses to grow so you will have more growth in the beginning and less as the saturation drops. I like the idea of the experiment and would love to see the results but I believe that once you have determined the amount being removed you would be better off to allow high flow to make sure that the plants are getting as much access to the nitrate/phosphate as possible. When the plants take off you will need to prune them to ensure that they have space to grow.
A lot of people make the mistake of letting the plants grow till they fill the refuge and thing that volume helps with "cleaning" the water.
The other one is your idea, they think that the plant de-nitrate is the same as an aerobic where it is ideal to have the water come out the end at no nitrates. With plants however you will have good growth at the water entrance and less as it gets "eaten" by the plants at the far end you will have minimal growth on the plants as they are not getting the rich fertilizer.
I'm not trying to talk anybody down I'm just trying to help people understand the biology of the system and why it works. I'm more then willing to answer more questions... or if you want me to shut up I can do that too... I want to see how this experiment turns out
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One question... what lighting are you using? (plants need; CO2 provided by the bubbles faster is better... I would also put the bubbles at the bottom of the tank, nutrients provided by the fish nitrate/phosphate, and light, in order to grow well... more growth means more nitrate reduction)