Cool find! I am interested in what you find out. I have been thinking on something like this for my numerous small tanks.
The GFCI is a great idea. Any time you have water and electricity together, well, thats a bad thing. The GFCI will help protect you, your house and your fish from a possible injury.
As for the lighting, you can go any number of ways. You can use a single bulb over each compartment or you could use a longer set of lights covering the whole length. It would mostly depend on what you plan to do with the individual sections. I dont think I would use a jar though. I think a glass or acrylic sheet between the light and the water would suffice. A jar would trap any heat from the bulb, or worse- it could trap water that condensed or splashed into it.
For the plumbing, may I suggest a visit to your local lfs
and see how they have theirs set up?
The biggest problem with a setup like this with a single sump is that an issue with one tank will quickly spread to the other tanks that use that same sump. I would definately keep one part away from the others as a quarantine tank to prevent any diseases from a new purchase or a sick fish from getting a foothold in your entire system.
Good luck and keep us posted!