Re-homing Z: trouble resuming a print, what if I can't re-home?
Was able to make it happen, though I fear that I may have misjudged exactly where zero was, but we'll see in a few hours.

Anyway, for those who may come along later, here's what I figured out:

I already knew what my X, Y, and Z positions should be.

I kept the heat on for the bed so the print wouldn't shrink and break loose.

With the plate lowered and the print well out of harm's way, I homed X and Y. Then moved X10 Y0, a convenient place to work.

I set relative positioning on, G91, and moved Z up toward the print head until a business card was firmly clamped between the head and the plate.

I then set Z at zero, G92 Z0. Apparently Marlin (or Octoprint? idk) will let you print without homing Z as long as it knows where zero is.

I moved to print head to my desired X, Y, and Z positions and checked it against the model, which seemed to be right. I fear that I may have not zero'd down enough, so there will be a noticeable gap/weak spot, but I won't know that until I get more layers on than there are at this moment. I suppose if that's true, then I could just shave off the new layers and reset zero a couple of layers tighter.

The rest of the steps for resuming a print (editing the G-Code and what-not) are readily available in a dozen or more tutorials, so I won't cover that here.

Follow-up: yep, left too much room. I shaved off the new layers (having a kiridashi really helped there) and re-zero'd, but this time, after doing so I entered my initial Z value so that the nozzle touched the model and adjusted from there. Note that aesthetics aren't an issue: these are mechanical parts to all they need is to not snap in the middle. Having a seam there isn't a problem.