Good luck with that - the poor man's cuben fiber!
A few thoughts:
1. You won't be able to sew it worth a darn because it gums up the needle.
2. It will be very difficult to make a pack without sewing.
3. I doubt you'd be able to glue it like cuben.
4. Silnylon will be lighter IMHO. The little tape I used on my tarp added at least 2 ounces (~50% of the LDPE weight) as I recall.
However, I'd love you to prove me wrong so have at it!