"I'm thinking of trying to use some of that 1/8 braided Dacron rope they sell at Cooke or Piragis outdoor. Holds 450 lbs and weighs .05 oz for 80 ft."

That's more or less what I use - it is  1/8" Dacron Kite line - 500# test
does not seem to stretch.    Color is white, so easy to see.    

just google for kite supplies and lots of online dealers should pop up


edit - my notes say my rope kit and pulley weighs 11.2 oz.  I use two ropes to go from pulley in center to each of two trees - easier to throw and coil two seperate ropes as compared to one longer rope (note: I use a lighter heaving line to get up over the branches - I'd never break that 500# line if it got stuck); a third rope runs through the pulley to hoist with - gloves do help with the small diameter line.  I get away with a single pulley when solo - my food for this year weighs 16# 5oz for 12 days - add in the pack and I'm still only hoisting 17# 3.6 oz max at the start of the trip. If I go with my brother, and double the weight, I'll often use the same setup, or may add two more pulleys.

the dacron line knots ok, but it won't knot as tightly as stretchier nylon line.