I've been wondering if pure dyneema braided fishing line would be the ultimate for light weight guylines/cord since it's cheap, unbelievable light and reasonable strong.
On eBay you can get braided dyneema/spectra fishing line in a variety of weights, but 100lb test might be the sweet spot. You can buy a 100 meter (330 ft) roll of it for $6 Buy It Now with free shipping. At 1/45" thick and with a breaking strength of 100 lbs it seems adequately strong for guylines. Compare this to BPL 'Aircord 1' which is 1/25" thick spectra with a breaking strength of 216 lbs and a cost of $17 for 50 feet. Aircord 1 is 0.004oz (or .11g) per foot and this cord would likely be no more than half of that. Honestly, there wouldn't be much of a weight savings over AirCord 1, but it is lighter and this stuff is a lot cheaper plus it might be more visible due to the apparently bright colors.
It also comes in a range of colors including bright yellow. Just search 'spectra fishing 100lb' on eBay.
What do you guys think? It might be slightly annoying to handle because it's so thin? I think it would be fine for applications where you aren't regularly tying knots. It would definitely be too thin for bear bagging. For $6 you could replace every guyline and drawstring in your kit.