Dear Willys:
This is a really good post, and I hope you keep us informed on your progress with this project!
I've used dropcloths for camping, mostly well used. ( I'm a painter ) Never was concerned with the specific weight of canvas material, heavier is better for painting, unless it's on sale! I just grabbed what was handy, and went. The lighter stuff worked fine as far as shelter, ( very simple shelter ) but then I always use a plastic truck tarp as a fly over everything for shade as well as rain protection.
Now if your going to go thru the trouble of making a tent, yeah, I guess a few questions are in order. Since I've never sewn up a new tent, your question really has me thinking. May I ask, are the heavier weights of canvas really necessary, and if so why? The relatively lighter weights seemed to work well for us, they provided privacy and kept some of the nighttime chill out. The plastic fly did the hard work.
Please understand, I'm asking questions in the hope that someone has the specific answer or, at least some thoughts. However, my own personal observations, as well as vast experiences are based on very sound scientific principles. The stuff was cheap because it was expendable, and it was there to use.
A commercial awning maker is someone that comes to mind as a source of info and experience for such a project, maybe some helpful tips as well.
Please do keep us posted. Thank you! Woodbutcher