What is involved in converting an IGH into a mid-drive? I've seen many examples, which usually have the sprocket in the area where a spoke flange would be.

Are there special sprockets that bolt to the flange, or, are only certain hubs available with shells designed for use as mid-drives?

I've seen cogs that bolt to the flange, but I don't know off hand where to get them. You can buy Sturmey Archer hubs already set up as gear boxes, complete with reverse.
https://www.sturmey-archer.com/products/hubs/cid/6/id/46

You can make or purchase cogs that have the standard disc brake (a.k.a. 'disk break') six-bolt mount. With one, any std. disc IGH can be used as a mid-drive if the chain line can be arranged.

pretty much how it works , primary chain goes to the hub's cog,
a secondary chain is attached to the hub shell..

Rohloff uses a 4 bolt pattern for the disc mount, and there are chainrings for that pattern in common,
then the second chain goes to the left side of a lefty hub.

Another German company, Pinion, has further developed a mid drive to have frames built around it..
It internally resembles a small motorcycle transmission, 18 ratios..
1 builder:https://www.en.tout-terrain.de/bicycles/silkroad-xplore/

the Sturmey (and Shimano) 3 speed hubs with a Cog on the hub shell
are used on adult upright tricycle builds .

***

Wabac in the JFK early 60's father of a friend , used a similar thing
though it needed to be fabricated in the shop, [he made prosthetic limbs as a business]

On a regular DF bike the hub went up at the top of the seatstay,
second chain went to another 3 speed in the hub.

They are known as "Tomicogs": https://tomicog.blogspot.com/

Contact United Bicycle Supply or Bike Tools Etc. about availability of the trike/quad hubs:
https://www.unitedbicyclesupply.com/i...s&tc=Quad_Hubs
https://www.biketoolsetc.com/index.cg...s&tc=Quad-Hubs
I think they're the same company- UBS is wholesale, Bike Tools Etc. is retail.

UBS shows out of stock. They are low on most hub models, waiting on a container they know not the whereabouts of last I spoke to them.

Yeah. When I last talked to them, they said they often don't know what's coming until they open the container.

Umm... DO WANT!

But seriously. Out of my price range. But I still want it.

Actually, a bottom bracket transmission rather than a mid-drive (that is, as the term has been used since the 1880s).

That's not what Boone, Phil, or Velo Solo call theirs, but whatever.

when I bought mom a khs trike, the mid drive was an ordinary nexus 3spd hub with a cog welded to one flange. Got a tig welder and some aluminum cogs?

Or Know a company owning a CNC laser cutter..
Ive seen some where there were a bunch of holes made in the cog,
to pop-rivet to the hubshell's spoke holes.

I'd just buy an off the shelf Sturmey Archer gearbox. They're mostly intended for use on delta trikes so can be found with a reverse gear feature- when you pedal backwards, you move backwards in low gear.