I just read an article on 2 and 3mil plastic used as a tarp on backpackinglight.com. Good article and will help a lot. The guy said he's used them in the field and it works fine, but the biggest problem is making tie outs. He used duct tape then grommets and on the ridge did the rock in tarp with line around it.
3mil plastic is supposably 2.0oz per yard and 2mil is 1.4oz(about the same as silnylon) so 6 mil may be a bit heavy. Assumingly 4oz per yard.
After reading it I want to make one as well, mainly for backyard use (to perserve the life of my good tarps) and as a cooking tarp with multiple people. I don't think I'd ever rely on it as a main tarp without a backup plan. Good luck, I'd love to see how it works for you.