OK I did it. I took my plane out to the side of the house, using fine white thread mounted several inches from the top of the cowl on a wire, with the plane facing into a light wind such that with the engine off, the streamer didn't favor one side or the other.

That was a little hard to see on photos so I took it to the most sheltered area outside my house and used red ribbon. The wind reaching the area was very light and variable. again I positioned the plane so the streamer didn't favor one side or the other before starting the engine.

I mounted the streamers on a 5" (or so) piece of wire, holding it well away from the slipstream core, because I reasoned that the effect would grow progressively stronger toward the propeller tips, since the tangent velocity of the prop and therefore the spiral is MUCH higher there, which might explain why thread taped to the side of the fuselage (or oil on the fuse) does not show the effect as conclusively.

As you can see, the spiral component is unmistakable, especially considering the engine is mounted with right thrust, which would tend to decrease the angle the streamer makes with the fuselage centerline. I even altered the position of the plane for some of the photos (the head-on picture is one) so that the light prevailing wind tended to decrease the angle (blow it back toward the fuselage), and at relatively high power settings (1/3 throttle), the streamer still spent by far most of its time on the right side of the centerline as long as I didn't angle it with too much of a crosswind. With full throttle, the crosswind might not have mattered much. The streamer did come over to the "wrong" side a few times, but only briefly each time. The streamers clearly and overwhelmingly favored the right side of the fuselage centerline. This can also be seen in the videos I shot.

You can't see it in the photo, but I was protected by a pipe while taking the head-on picture, in case the plane broke loose. I always try to be safe.

I never exceeded about 1/3 throttle because I was nervous about the stake coming out of the ground. It probably wouldn't have, but I didn't want to stress the tailwheel either, and I didn't have a spotter.

I have a few short videos too. I'll try to post them and edit this thread with links when I get that done. Sorry about the tiny, low quality videos; cable and DSL are not available in my neighborhood yet. I think I'll only post the smaller videos with the red ribbons for now. The first two vidoes with the white thread show the same result under less favorable conditions (more wind), and are much larger files.

Here are two little videos:

http://media.putfile.com/Spiral-Slipstream-1
http://media.putfile.com/Spiral-Slipstream-2

P.S. Here is due credit to aspectratio for jogging my memory about the tangent velocity of the spiral as a function of its distance from the core.