What I couldn't find?
How the caraco closed.
It's truly unlike other garments from the period. Most garments either utilize a stomacher, which the wearer pins or laces a gown or jacket over, or the garment simply closes at the front. Variations include false stomachers, and items with stomachers often utilize robing to make pinning easier; closed fronts became more fashionable than stomachers, generally speaking, by the late 1770s. Regardless. In short, other 18th century garments very obviously close at the front, in visible ways.
But caracos just kind of....float.
Of course they must be fastened *somehow*--and descriptions in fashion plates confirmed that some were fastened "midway" and some "all the way down" (my own crappy translation, of course), varying just how "floaty" the front looks.
How they fastened, however, was a mystery to me. I deduced that, in order to achieve the flouncey, floaty look of the front, there had to be a separate placket inside for the actual work of closing the garment. That would let the trimmed front do its thing. But how? Hooks and eyes? Pinned?
As it turns out, laced.
More images and info about this piece here: http://whitakerauction.smugmug.com/Spring2013/Clothing/LOT-704/
After considering this, it makes sense. This allows for the front of the garment to still be structured underneath the floaty outer layer.
I had taken the wrong guess before finally finding this image--I had created a placket for pinning the garment. I'll be redoing that after its first wearing, especially if the pins do not work well. Of course, I also hesitate to assume universality from one garment--spend much time looking at extant garments and it becomes clear that sometimes there are exceptions to the rule, sometimes the rule is more of a guideline, and sometimes there is no rule. But with nothing else to go on, I'll stick with this image's demonstration of a laced placket front closure, barring future discovery of other methods.