Pignut
I don't know if you are aware, but Endicotts in Exeter do a conversion of a Dutch bivvi bag with a zip down the side.
I have been thinking about doing this to a bivvi bag myself and what I was planning to do was sew on a flap of goretex to keep the weather off the zip at the join. I also plan to treat the zip and stitching with a seam sealer, I use Mcnetts Seamsure myself, but there are loads on the market.
I've just got to get a decent zip and I am ready to have a go.
You could always ring Endicotts and see if they would give you a few clues, they are usually pretty helpful.
Regards
Phill