Howdy.
I just read your update to your GWC diary page. Let me make another
untried suggestion. You can use graphite, as in the form of carbon
paper. Perhaps you could just rub the paper onto the wax sheet, and heat
it slightly to embed the carbon. When my friend put together the
non-functioning GWC, he used some rubber cement (hexane) that I had
bought for this purpose. The thing is though, if you use cement, or any
other adhesive, you now have

metal - glue - dielectric - glue - metal

as opposed to

metal - dielectric - metal

And according to the reports, compressing the layers during construction
is very important. So, I would imagine that adding glue (even only a
fraction of a millimeter) may affect the desired outcome.

As for using carbon powder, check out the following links for some

ideas:

http://www.physics.udel.edu/wwwusers/watson/scen103/less-copier.html

http://www.physics.uoguelph.ca/summer/scor/articles/scor54.htm

These links describe how a photocopier works. I was imagining having
toner powder and sprinkling it onto the wax paper, and then heating the
paper just enough to cause the particles to embed themselves into the
paper. This is just like a photocopier, except now you can make the
carbon layer THICK, and not just a surface coating. Well, maybe. I don't
know if this will work. And, then, you need a heat source. I supposed
taking anything hot and connecting it to some foil will transfer the
heat.

Actually, I'm liking the india ink approach even more. Just brush and
dry.
As for your concerns about it not being a good conductor...Well, yeah,
it's true that carbon is pretty resistive. But if all it has to do is
act like a plate of a capacitor, then it will be good enough. Little
current should flow through the dielectric, anyway. Then, again, making
the plate out of carbon will add a lot of additional resistance,
creating a larger RC time constant. This may affect the results, too.

Ah, nuts. What is needed is liquid metal that can be brushed on (as sold
by Newark Electronics), or a metal powder that can be applied and then
set. Actually, what would work is metal sputtering in a vacuum. But if
you go to that much trouble, you might as well do it by cutting strips
of foil by hand.

Wait a second. What about using your glue idea like this. Take some
powdered metal (somewhere) and add a little bit of glue to it and apply
it with a brush to the dielectric. This is similar to Newark's liquid
nickel. Sounds expensive.

I'm reminded of etching PC boards. PC boards have copper laid down on an
insulating board. To etch the board, ferric chloride is used. I wonder
what chemicals could do the opposite...