This active, spiraling coordination, maintaining the internal connection, can be trained through chansi gong, silk-reeling exercises, as a separate practice apart from the solo taiji form.
Interesting, because I would claim the opposite. In my experience, maintaining the internal connection is what creates the spiraling coordination. One can disturb this spiraling e.g. by active muscle usage, but in the end when the internal connection(s) is/are there, the body wants
to move in a spiraling fashion. So it's more about allowing the body to do so, then about actively coordinating the spiraling.
[...], I can only suggest that the heightened proprioceptive awareness of internal connection and breath, central equilibrium and balance in (slow) movement, might fruitfully carry over to solo aikido basics and improve the connected quality of movement.
The above seems to suggest that the only thing practiced by silk reeling is proprioceptive awareness. This would mean that everyone already has all physical qualities needed to perform silk reeling correctly, save the proprioception part. I do not think that's true. The internal connections need to be conditioned and the coordination to move in accordance with these connections needs to be developed.
To expand on the above points, here's how I would explain silk-reeling.
The body has a front side and a back side. The front side consists of the insides of the arms, the insides of the legs and the frontside of the torso. The backside consists of the outsides of the arms, the outsides of the legs and the backside of the torso. (For more detail, take a look at the main meridians of the body. And for a slightly different view, the pictures from the book by Chen Xi that were posted.)
Once can contract the frontside of the body with the dantien/hara as center to 'close' the body. One can also contract the backside of the body, again with the dantien/hara as center to 'open' the body. This opening and closing of the body can only be done if the internal connections of your body have been conditioned sufficiently to guide the movement. And if you allow these connections to fully define your movement, you'll get the typical spiraling movements of silk-reeling.
The main point I'd like to make is that first of all silk-reeling is about physical development. It's a whole-body workout. It's Chen Tai Chi calisthenics.