scarf,
I don't have a definitive answer, but I think it likely that green coffee beans wouldn't work well. When brewed, ground green coffee beans produce a very bitter liquid. There's some flavor, but not much, and it's overwhelmed by the bitterness. The flavor that does come through tastes more like tea than coffee.
Way back when, Peets' Coffee in San Francisco began the movement toward darker-roasted beans (via longer and/or hotter roasts, typically called French or Italian roast levels). That's been carried on to this day by Starbucks (sarcastically called Charbucks by coffee geeks). Dark-roasted beans have a sheen, which is the oils being drawn out from the interior onto the surface of the bean. Dark roasting intensifies the essential coffee-ness of the flavor, making it more bold and pungent, but also destroys the delicate nuances of the beans' flavor elements. Everything gets caramelized bluntly into burnt sugar.
Espresso aficionados don't roast dark. We roast our green beans to a medium-brown color with no visible oil (called City or City+). Doing so preserves the subtle flavors of fruits and flowers that are imparted by the soils where each coffee is grown. That's the roast level I used for my first (failed) maceration. I roast to the beginning of what's called "second crack" (first crack occurs midway through the roast and sounds like popcorn popping as the water inside the beans is heated to steam and cracks the bean to escape; second crack sounds like Rice Krispies when the milk is poured onto the cereal; there is no third crack, just charred bean death).
I roast in 12-ounce batches, so I could conceivably roast a batch even lighter (called a Cinnamon roast), but I'm inclined to think that the heat applied during the maceration is more likely the problem than initial roast level before grinding. I could try the microwave method followed by a long cold maceration, but I'm dubious about that. My two-day cook didn't have deep, rich flavor, so I'm doubtful that a short burst of heat would do the trick.
I'm not convinced that heat is the problem, although it certainly could be. I wonder if the espresso grind I used might have been too fine. For instance, French Press extraction uses very coarsely-ground beans. I'll try a coarser grind for my next maceration, and Central American rather than Ethiopian beans.
Freshness could have been an issue with your batch, but not mine. I used coffee that was seven days past roast and ground literally a moment before adding PG/VG. My extract doesn't taste stale, just a bit weak and metallic, rather than the rich and deep coffee flavor I was hoping for.
Our liquid might be a culprit also. Coffee for drinking is extracted into water. We extract into PG or PG/VG. I wonder if the aqueous carrier might involved in producing the metallic flavor I got. Maybe I'll try two extractions next, one all-PG and the other all-VG.
Finally, a very different option would instant coffee (presumably high-quality, if such a thing exists), either alone or mixed with with freshly ground roasted beans. As much as I hate to admit it, instant freeze-dried coffee might be the easiest and best solution. I probably won't try that, though, because I don't have any instant coffee, and the thought of buying some makes my spinal cord shrivel up.
