Since I asked the question, here is what I was thinking of doing. First, I want to minimize the amount of rework to the dinette seats and supports. Therefore, I plan to buy some 1/4 in thick high quality aluminum plate that will fit under the seats a few inches with whatever width I want coming out to the center isle. Thin aluminum would not require much modification to the existing seats or cushions.
I would support the aluminum extension by screwing two end caps for 1.5 or 2 in. schedule 40 PVC to the bottom of the plate, then buy the PVC for the legs.
Using this arrangement, I could remove the metal plates during the day and the PVC legs. For the center table section, I would slide it out to the same width as the extension and place an aluminum plate on the other side that is supported as well as hold the table top from tipping. Then purchase pieces of foam and cover them to fit the extensions. I would probably screw a half round piece on the edge of the extensions to hold the foam.
Doing it this way makes the extensions very portable and stowable during the day. The foam can ride in the V-berth during the day.
Unless I hear of someone else's idea that is better, I will build it and post pictures here.
Terry