Even better: I have easily buried 1/8" shock cord within single-braided hi-tek line on hand, both of them (Spectra and Vectran) far stronger than needed.
No accurate scale here for small weights to measure the savings in mass. I am surprised that for so much concern over paring weight, the inclusion of more-than-minimum rubber shock cord in the system is not given more attention. Not only is it dense and heavy, it isn't even rated for strength, which given its light cover and rubber core cannot be very great. How much or little strength is needed to hold up a 2 lb UQ, even when that is done by stretching it out at a 30 degree hang angle?
You only need lengths of shock cord twice as long as the range of adjustment required by the system:
Calculation: Pre-tensioned at 30-50% of its length and maxed out at about double the length. So usable range = (100%-50% = 50%)
Using just five feet of shock cord might save most of the mass of another twenty feet of a full-rail shock-cord suspension since twenty feet of 200lb test Dyneema weighs less than 8 grams.
To be fair, JRB (and others) don't have channels and rails in the first place. The JRB shock cords for both ends, weigh < 1/2 oz .