From the limited knowledge of a 14 yr old, i have to point out a few "flaws" in the idea.
1.) Wouldnt each car need its own address? Im just thinking that you would run out of adress space on the DCC system fairly quickly, or you would need multiple systems.
2.) How do you plan on concealing whatever mechanism is used to physically open the coupler? the trucks of the car would most likely be in the way i would imagine.
Some ideas for the flat car/well car concept.
Couldnt you take out the weights on the underside of the car, and have all of the equipment up in there? Like the decoder, the battery and the wires, i would imagine all of that would be sufficient weight, and if it isnt there ought to be room left over to add some.
What type of batteries do you plan on using, something like a watch battery would be best for concealment i would imagine, but something like a 9v would be easiet to wire. Maybe it would be easier to practice on a larger scale?, like O scale boxcars or something?, then downsize from there.
Like i said i am only 14, but ive taken years of AP math, sciences, social studies, and english classes, as well as taken part in multiple courses in nanotechnology and i have an electrician for a father, so i hopefully i can help alittle bit