I had an Adventure Medical Kit Emergency Blanket lying around, so I thought I would try it out as a tarp. At 4.5' x 7', this is too small to use as a hammock tarp, but I figured it was worth trying, since AMK makes a slightly larger (5' x 8') blanket that could potentially be used as a hammock tarp, and it certainly seems possible to combine two of these to still have a low cost lightweight option for a tarp.
Anyway, for my test, I set up a ridgeline and used sheet-bend knots to tie off the corners. I pulled it fairly tight, but it could have been tighter. If this were to be modified to be really used as a tarp, it would need to have reinforcement for the center line, reinforced corners, and possibly a grommet in the middle of the sides that would allow a third tie-out on each side.
It took about 15 minutes to set up, and I left it up for about half an hour. It did not tear in that time. When I untied the corners, the material had clearly weakened. Part of the metallic underside had rubbed off, making it look somewhat thin. While up, it worked to keep rain off of me, though it was just a light sprinkle and not a downpour. I would not have wanted to set this up in high wind (though, really, who would want to set up any tarp in high wind?). Noise was not particularly problematic.
Overall, I think if a trip <3 nights was planned, this would be a viable option without reinforcement. I think it could provide needed comfort, but I would hate to trust my life to it.