Thanks for the input, kylea, The Dog & Claus!
I decided to go with my original plan, and it worked out very well. Here's a recap in case it will be useful to others:
I had to repair a short section (~6 cm ~2.5 inches) of seam that came apart on our spinnaker, in an interior seam ~155 cm (~61 inches) from the head.
It appears that the spinnaker panels were attached to each other at the seams via a strip of adhesive ~12 cm (slightly less than 0.5 inch) wide, after which the seams were sewn with the large stitches in a zig-zag pattern. The adhesive appears to provide essentially all of the shear strength to hold the panels together while the spinnaker is filled; the sewing just keeps the layers from peeling apart during handling.
The seams show no evidence of any tape backing, only adhesive. Thus, it seems apparent that the adhesive was applied via transfer tape. A web search for same turned up Innotec of Wisconsin. Their sales rep suggested their TH253 transfer tape, which I understand they sell for making spinnakers. They cut it to width as requested (12 mm v. 0.5 in. = 12.7 mm in stock), and mailed a roll to me for a very reasonable price. I think 3M might make a comparable product, but I was not able to find anyone to sell a small quantity retail, never mind custom width.
For surface preparation, I used isopropyl alcohol (IPA). I'm concerned that any stronger solvent, such as acetone (polar) or mineral spirits (non-polar) might be harmful to the material.
I cut a piece of the adhesive transfer tape to the size & shape of the gap, then cut it in half to facilitate positioning the ends. I then used tweezers & popsicle sticks to position the tape, rollered with a clean wall paper seam roller to make sure the adhesive was stuck down, removed the backing with the tweezers, positioned the top layer, & re-rollered.
Then my wife re-stitched the seam by hand, using polyester thread. Biggest problem: The adhesive kept gumming up the needle, requiring frequent cleaning with IPA.
The repair has held up fine so far, through ~5 hours in largely ~15 knot winds. This was with a fair amount of flogging, shrimping & getting hung up on the bow, which we plan to reduce as we gain experience.
So, all in all, I'm happy with the repair, except for the amount of time it took to figure out how to do it! I probably spent half a day researching the subject. But, with the benefit of experience, I think it would take ~1 hour do to a similar repair again, less timethan to take it to a sail loft unless there is one near by, & certainly less expense.