OK Ray . . . here goes another trade secret!
I use Renia Klebfest neoprene glue to fix zips before stitching. The tubes have a narrow nozzle, with which it is possible to make a 4mm line of glue on the leather, and a similar line on the zip tape. Allow the glue to dry. When it's time to complete the assembly, the glue can be re-activated by gently warming, and it then becomes a powerful contact adhesive. Press the zip into place, and it'll stay there while the stitching is done.
I tried double-sided tape many years ago, but (1) it's too wide and (2) it clogs up the machine needle unless the thread is run through an oil-bath, which then leaves oily marks on the leather. Super-glue is an absolute no-no! It soaks into the leather and turns it into something resembling concrete!
Renia Klebfest is available from http://www.tcolledgeandson.com/ in 60g tubes.
Hope this helps!
Terry