James, I partly agree with you that if you're right handed, a left zip is easier to use since you reach across your chest with your favored hand to zip or unzip, rather than using your less coordinated hand. And of course in a tight fitting mummy, it's nearly impossible to use the same had that's on the zip side since you don't have enough room to move your arm up and down. But on the second part of what you said, it shouldn't make any difference whether you use a quilt since quilts don't have zippers. Now if you meant using a quilt in a bivy, well then I still don't see how it differs from using a mummy bag. The majority of the zip is on the opposite side and only the final closing comes across the face closest to the hand your using to zip and unzip. I would think that if you're right handed, you'd still want to unzip across your face with your right hand moving the zipper away from your right side, then down the side opposite your right hand, the same rational for using a left zip mummy bag if you're right handed. Anyway, that's my thinking, quilt or no quilt.
The other reason in the old days that I recall left zip sleeping bags were more popular (aside from the fact that they're easier to use if you're right handed and the majority of people are right handed) was that they used to make bags to pair or zip together. The left zip bag had the draft tube on the top of the zipper and the right zip bag had the draft tube on the bottom of the zipper so that when you zipped 2 together, there was a draft tube covering both zips (hope that makes sense). So the theory was that for everyday single use, if you had a draft tube on the bottom of the zip, it would have more of a tendency to fall away from the zipper thus creating a cold spot. But, if you had the draft tube on the top, it would naturally hang down and cover the zipper area. All the major bag makers of the day (30 to 40 years ago) used to make bags this way. Anyway, that's my thoughts on the matter. Thanks for your input!
My guess is that someone at Katabatic gear is left handed, probably Aaron. Notice in his picture he appears to be using his left hand to take the photo. He probably always used right zip bags and never thought about why. When making the decision to only offer one sided zip on the bivy, it was probably natural to just choose what you use. Just my theory on the matter.