I think if I were going to do this mod, here's how I'd go about it:
Sew the zipper into the hammock, with both halves together. That way you KNOW everything will be lined up when you're done. It'll be kind of a pain because you'll have to get the machine in through the slit, but I think it's doable. T-Back said he did his that way.
Once it's sewn in, THEN open the zipper a little at a time and cut the netting under the center of the teeth. This should leave some material on each side.
Roll the extra material over and sew a couple more lines. I think I'd add a flat strip of grosgrain on top too, for looks and strength.
I don't think it'd be THAT hard of a project. I may get some zipper soon and try it, especially since I don't use my HH much anymore. I think if the mod were successful, I might be a lot more tempted to hang it than I am now.
"Physics is the only true science. All else is stamp collecting." - J. J. Thompson