So I just picked up a cheap Spin Trispoke for one of my bikes, and it looks like it's missing a piece of the hub.
On the drive-side, the aluminum doesn't stick out nearly as far and there's a threaded area, as shown:
I'd assume that this is so that the wheel can be used for the front or rear, and that there's another piece required to use it as a front wheel (or rear, but that's not what I want.) Is this correct? And if so, does anyone have close-up photos of a piece that fits into there? I'm wondering how much work is involved in making this wheel complete.
I'm reasonably certain that Spin wheels had hubs custom-made for their setup, so you'll have to find another Spin wheel to cannibalize. This might not be tough- Spin wheels were known to turn into triangles under the influence of high-pressure tires. This may also be the reason the company is no longer in business.
...does anyone know if it would be possible to use a Surly Fixxer to turn it into a rear wheel?
Not really. There are two main hurdles:
1) the internal thread showing on the pic is probably bigger than the internal thread that the sury fixxer is supposed to engage.
2) even if by a stroke of blind luck the threads would be compatible your hub hasn't got the splined interface which the fixxer is supposed to fit over. W/o those splined bits interlocking you'd have a "suicide" conversion, meaning that the cog could come off if you put backpressure on the pedals.