Well you can try it, but I can see a few problem. First is one 75 gallon aquarium air
pump wont be anywhere near enough air for the amount of plants your planing. Especially for the large ones like tomatoes. You'll want a high volume air pump, something like this:
Super Luft Pump - 3.9 psi - 66W (Output 2.3 CUBIC FEET PER MINUTE). Second a PVC tube with holes drilled in it wont be able to create all the tiny air bubbles needed like an air defuser (air stone) can, I've already tried it. Bottom line you'll want the air bubbles to be as small as they can be in order to create as much surface area between the air bubbles and the water. That's what allows the oxygen molecules to be absorbed by the water. That also allows them to be distributed thought the water and through the plants roots much better. You might try soaker hose (like for drip irrigation), it's my understanding it works well for a air stone replacement. Providing the tiny bubbles, it's also flexible so you can cover the bottom fairly inexpensively. I haven't tried it yet, but I'm told it dosen't clog like the the regular flexible air stones do.
Next is the weight. You said you didn't want your table to weigh 1000 pounds. A 8 foot long by 4 foot wide table that is 6 inches deep in water will be heavier than 1000 pounds. That will be 16 cubic feet of water. There are 7.48 gallons of water in a cubic foot. 7.48x16= 119.68 total gallons. A gallon of water weighs approximately 10 pounds. That means your table 6 inches deep full of water will weigh about 1200 pounds (without the plants). it can be done, but make sure the base is very sturdy.
Next problem is the top section you plan to place the plants in. I don't know how you plan to construct it, but there are a few design issues you will want to think about for maintenance. First You'll want to make it in sections instead of one piece. If not you will have an extremely hard time taking plants out and replacing them. The roots will also become intertwined with other plants, and cause you problems when it comes time to move them around. Also if you made it one piece you will never be able to lift it with the weight of the plants in it, even if you could or had people help you, it would collapses under the weight of the plants (when growing large plants like tomatoes, beans, peas, peppers and herbs etc.). Witch leads me to the next point, you will want to create a support system for the top spanning across the water to support the plants weight as they grow heavier daily.
Another maintenance issue is
nutrient changes. You will want to at least build into your table top water culture system a "draining system." Otherwise nutrient changes will be too difficult to do. But you should really have a table to place the plants on so you can clean the water tray as well, if not you wont be able to clean it if there is a problem without killing your plants.
But if it were me and I was going with a large water culture system like that I would also use a secondary reservoir (with at least one third the water volume of the system). Pump it up through the system, and have it overflow back down to the secondary reservoir. Not only would that provide better water circulation, but make nutrient changes much easier. Instead of completely changing it, I would just empty the secondary reservoir, clean it and fill it with fresh
nutrients, and only clean the main system if/when it needs it. Then let the fresh nutrients cycle through the main system. Basically that would replace 1/3 of the volume of water with fresh nutrients each time, and I wouldn't need to worry about the water level in the main system either, and/or how to add replacement water that the plants drink up (simply add that to the secondary reservoir).