Thanks.This is what the latch looks like, which may help.
I fixed this today and will post some notes in case anyone else runs into the same problem.
1. Unscrewing the two rearmost screws holding the rear fender liner (from beneath) allows to one to (gently) twist and work the liner down far enough to fish the lock and cable through.
2. Once the seat is open, remove all of the rear bodywork except for the fender liner. This means pulling the luggage rack/bag mounts, etc. This takes a long time but it's just a lot of screws and some bolts. Eventually you will get to the "box" above the fender liner where the lock is located.
3. My lock bracket had just broken at the two (very shallow) plastic posts which normally hold it to the rear plastic "box". It's a flimsy design and I expect others may run into this problem over time. The lock assembly itself (tumbler, cable arm, etc.) mounts into the lock bracket with two screws.
4. I used a two-part epoxy to glue the bracket back in place not just at the broken posts but across both lower plates at the bottom of the bracket. I then was able to drill one hole from above (drill access is tricky) and installed a bolt with washers which passes through the "rear box" (near the lock), the bracket and the lock assembly. That should make things pretty secure.
Sorry that I did not make pictures. I am in a bit of a rush getting ready for a trip.