Last post of the day! I am getting better results. My modification of your script does seem to correctly tag the original JPG sidecar. I am still having trouble programmatically selecting the top item but I just noticed something. For some stacks, I am getting red out-of-sync indicators while stacking. I cannot isolate the cause of the problem. I've tried to resolve by writing metadata to disk but the red indicator returns on the next attempt to stack.
The only resolution is to unstack, select all members, remove from database, then scan for new files. Although this sucks, I will live with it to see if I'm in a steady useable workflow.
I suspect that parts of my problem have included: multiple attempts to stack without completely clearing database entries for the stack members, not carefully noting when metadata synchronization issues crop up, not understanding the way matches work in compound (comma-separaterd) lists, not knowing that I could add script to the tiny little input field for Custom File Masks, and getting edge condition errors without identifying all the components of the problem.
I repeat: this problem is not yet fully resolved but it seems close enough to be workable. Now on to deleting and recreating all stacks in my database. If any documentable, reproducible problems crop up, I'll report back.