Thanks for the links on Tyvek paper, gonna check that out. I sent an email to them asking about getting a sample sheet to test before buying a whole pack.
I've tried the Nat Geo 'Adventure Paper' (which I suspect is repackaged Rite-in-theRain) and while it is indeed much more rugged than plain paper, I found that HP inkjet ink would smear a bit (enough to affect legibility) if it got damp. It is also very thick.
In the time since I've used either a ZipLok bag or the Seal Line HP rolltop map case. These work well but the problem comes when you're doing nontrivial navigation and not just following a clear trail. A case makes it very inconvenient to mark the map as you are working in the field from point to point.
I'm going to try the Tyvek paper and see how it does when marked with a micro Sharpie.
I suspect the Tyvek paper will be much thinner and more manageable than the Rite-in-the-Rain/Nat Geo paper. At the link http://www.allweatherblueprints.com/tyvek-sheets-printing.html it lists the Tyvek papers as being the equivalent of 14# and 18# paper, which is even lighter than standard 20# bond paper. It's not the actual mass in grams, it's the flexibility for folding & rolling that matters.