I wrote this up for another comment, so I’ll put it as a reply to yours, too, so you get a notification about it:
In my time, I have played a fair amount of Dungeons & Dragons (and other such games), including “running” as a Game Master. The planning and narrating of locations is something I feel like would be greatly benefitted by having actual visual imagination. When I learned about aphantasia as a thing (and came to realize that when people talked about picturing something in their head, they were being a lot more literal than I realized was possible), one friend of mine wondered how I could do what I’ve done running those games, describing places aloud from my head, etc. without visual imagination. I said I don’t know, but that he should consider how much better it might have been if I could picture things.
As far as maybe getting you closer to my experience:
Look at some table or other small pieces of furniture near you. Think about what you are doing with normal visual processing - your eyes are getting simple brightness/color signals from incoming photons.
Those get sent to your brain, and a few layers of processing happen - this region is square or rhombus shaped, this region is darker, this part is narrow and tall. Another layer maybe predicts the parts you can’t see and gives you a sense of the table’s thickness at various points (legs, main surface).
One layer/process considers how the trapezoid shape you see as the surface is actually a square/rectangle, and the apparent width changes based on the distance of that part of the table. All this happens without you having to think much about it, and you end up with not just a simple map of “this square is dark brown, this trapezoid is grayish” but a sense of a whole complex object.
Now, take that multi-layered sense of the table and try to focus just on the physical shape of it, your sense of where each part exists in space. Try to “imagine”/consider the table as an object you sense the presence and shape of, and then also imagine it to be invisible. You still know it’s there, you have awareness of where you could walk without hitting it, how you could crawl under it, how far you should lower an object in your hand before letting it go so as to set it on the table rather than either dropping it or slamming it down.
If any of that clicked for you, that probably approximates the experience of non-visually imagining something solely spatially. Basically, everything the visual experience would tell you about the object, except now pretend it’s invisible.
Good to know, re: hypnagogia. I’ve occasionally tried experimenting with it when I think to, while floating near sleep. I’ve weirdly found that moving my eyes certain ways, or focusing my eyes to certain distances, while I’m near sleep (eyes closed) can make it feel very suddenly like I can see something. My intent was to find what seemed to work in that state and see if I could use such techniques while more awake, but if you’re right, then that presumably won’t be as successful as I’d hoped.