A nocturnal search with a flashlight revealed the culprit(s): Two small rats. While sucessfully evading my light they got into a fight with one another which slowed them down enough for me to follow their movement.
They exited the lemon tree via a branch-to-branch bridge into a nearby oak. I've since removed that bridge and a few "almost bridges", but still found one rat the next night. However, that rat didn't make for the exit, instead it froze in place under my gaze. Apparently I have complicated rat logistics somewhat. The baited trap in the tree hasn't been visited yet, I should probably give more thought to the bait.
The bark damage has gotten considerably worse than shown in the photos, but the tree is still pushing new growth and blooming.
Thanks for writing,
bob prohaska