Would a front (single ring) tandem crank and chainring work well for a single speed?

yup

If you want to go backwards. The only way to manage LSD for a single-speed would be to get a BMX hub that has reverse threading for a left-side freewheel. And adapting that BMX hub to work on your road/mountain/whatever bike would probably require a bit of hub surgery.

Going left-side drive with a fixed gear, on the other hand, is way easier. Just flip the wheel and throw the chain on.

why not just put the spider on the normal drive side?

Reversed pedal threads.

Duh, forgot about that. you could swap your spindles on your pedals.

There's a seller on ebay using helicoils to reverse the reversed threads back to normal.

It seems a little fishy to me, but they're nice cranks, and I've had to talk myself out of them a few times.

that would get the right pedals on the right side, but he purpose of having pedals threaded differently is to keep them from unthreading. if they are reversed, the pedals will have a tendancy to work lose. unless all you do is backwards circles.

Blue _______ is better for that.

Install your pedals with a little extra grunt and blue loctite and they won't unthread, it's not a big problem. Lots of pedals are symmetrical, most non-clipless and speedplay would work without spindle swap.

I would sell it and get a new (to you) crank.

Tandem parts sell at a premium as they are specilised.

I just had a wacky idea for SPDs and other assymmetrical pedals for the cheap LSD: How about flipping the cleats on your shoes? Put the front side towards the back, and so forth. Granted, the clipping in would be a bit different (you would probably "heel in" instead of "toe in"), but I'd be curious to see how it worked. If/when I upgrade my shoes, I'll try this. Then I can have a LSD pair of shoes and a regular set of shoes.

If I use platforms that wouldn't be an issue, yes? Provided they were tightened well and I use loctite as suggested, right?

Sure. Eggbeaters would work too.