I saw a still photographer using a single octagonal softbox with a flash inside. It gave a nice soft diffuse pool of light for the subject. Has anyone used such a rig for video? The results were beautiful.
a softbox, yes.
a flash, not exactly practical. the octagonal shape might not be either. you might be happier with a square or rectangular softbox with a continuous light source, be it tungsten or fluorescent.
Do a search in this forum on softboxes--you'll find much on the subject.
If you put a hot light in that softbox, it will go up in flames. Still lightboxes are very differently rated for heat. Not sure if anyone besides PhotoFlex makes an octagonal lightbox for video. If they do, I have not seen it.
I've used hot-light ectagonal softboxes. They work fine, just the same as in still. The octagonal shape makes them flatter, so they take up less space for any given size. But the light doesn't quite spread to the edges as well. It's still softer/larger light for a given profile/depth than you'd get with a traditional box.
Just make sure anything you buy is intended for real hot-light use. I've had cheapie soft boxes (square ones) go up in flames (literally) b/c they were knock-off cheap-o ^%$$# that ignored the most obvious design principle of accesorizing HOT lights. So certainly be weary in that regard, but I don't think the heat dissipation of varying shapes is really cause for any concern. Making sure that it's from a reputable manufacturer designed specifically for hot lights with non-flammable materials IS.