I'm going to make a tarp/shelter/poncho and am going to order probably 5 yards of material from this website
Multicam Fabric

I'm leaning towards the lightest stuff they have that's still listed as waterproof.  It's not going to be a hard use/wear item it'll be stuffed into the bottom of my go bag for use as an emergency shelter or camo covering.

probably 500D that has been treated on the back for the best all around use.

That DWR should be fine. I mistakenly ordered some 500d fabric with DWR stuff on it - amazing stuff really. I could probably make a water bladder out of it; it's very plasticky.

Cuben fiber, if every gram counts.

Out of curiosity, why not just buy a multicam poncho? They're not that expensive and would serve a few purposes.

1) Can be worn, if you choose to (I realize you say it won't, but why not be ready just in case?)
2) A lean-to shelter can be made with one, some 550 cord, and a couple anchor points (trees, bushes, etc)
3) Can be used to wrap your pack in wet weather, thus keeping your gear dry.

70D is perfect for a tarp.

Im my opinion, nothing beats sil-nylon for a moderately hard use tarp.  Not so sure about backpacks, etc but there are those who will argue that all day.  Don't wear it whilst strolling through the briars and it'll go the distance.  Either the 30 denier in dark grey or OD green.  Note that I haven't actually used anything with cuben fiber.  Although it has always seemed to be pretty ultra light backpacker niche to me I suppose that's how innovation often looks..  

I'm not a DIY nerd but just happen to be working on a Mtn bike frame bag for an upcoming trip....  So heres a pic of 1 yd 500d cordura in coyote and 1 yd of 30d sil-nylon in orange.  Sorry I can't weight the 2 but the diff is substantial.  I've had the tarp for 7 years of pretty good use and its still GTG w a couple pieces of seam tape covering hard wear areas.  The nalgenes there for size.  Got it all from rockywoods.com.



hope that helps some,

spf

^^I like silnyl too. Give Cuben Fiber a try. you wont be disapointed.

I haven't been able to find any Multicam material anywhere else or Sil-Nylon (which I would love to use) in Multicam.

As for buying a premade poncho the only one I could find was about 70 dollars, and I could only find one supplier.  I have the ability to sew and install my own grommets and I think it would be a fun project for me and I can custom size it as well as make it out of the lightest material I think (or folks will think) will work reliably.

Edit- Interesting stuff.  Only 1.7 ounces per square yard
40D with "Nanosphere"

Nanosphere