I couldn't find any descritions of what exactly the "Flatline" is. If I had to guess, it is simply a flat (hence the name) cord, similar to other flat cords on the market used for guylines. It is probably comparable to BPL's Aircore 1, in that it is thin enough to be threaded through a needle. My guess is you could replace it with any of these other products and do just fine.
In looking at the design of the lifter patch, I'm wondering if the cord would work its way (tear) through the fabric once its put under pressure. I'm also not super keen on the idea of a knot being the only thing holding it there. 100lb breaking strength isn't bad, but it's not that high in comparison to most of the Spectra based products out there.