I have to admit I know nothing about the API, but yes, the date read is one of the things associated with the Goodreads "review." In fact, most of the editable features relating to a book in your list of GR books are part of the review: the shelf, the rating (i.e., stars), the date read, and the review itself; those all come up in the "Edit Review" dialogue.
If I want to add an actual written review, I'm likely to go to the GR site, not to use Calibre; there's no real reason for me to write reviews for Calibre, since it is only for my own use. But something I do very frequently (just to give you some idea of how at least one person uses Goodreads!), in fact every time I finish a book, is to change it from "to-read" to "read" and at the same time fill in the "Date Read" column (both custom Calibre columns) (if I were a big user of ratings, I might at the same time also fill in the ratings field). Since I also use GR, I am in the habit of making those same changes in GR at the same time, to the "read" shelf and the Date Read field, when I finish a book.
In my ideal world, the plug-in would permit me to make these changes in Calibre and then export them to GR, so I don't have to replicate them in GR myself. As it is, the only thing I can change with the plug-in is the shelf; since I can't change the date-read field (or ratings, though that's not so important to me), I have to go to GR anyway, so the plug-in doesn't save me anything in this particular situation (it is still very helpful when I add a book, though!).
I guess I would envision, at least off the top of my head, a UI that would permit specifying something very like the mapping the plug-in already has, but in reverse, and that can be applied on an individual book basis. But again, since I have no familiarity with the API, I may have no idea what I'm talking about.
I suppose an alternative - less appealing to me since I prefer to work in Calibre, but one that still would permit a single entry of information when finishing a book - would be to enter the info in GR and then sync into Calibre - but as best I can tell, the sync now is only on a shelf-by-shelf basis, not a book-by-book basis, is that right?
Anyway, I'm not sure I'm much help, but thanks for continuing to consider all this!