First try...

[BPTactical]

Quick question for guys with more experience - I'm obviously too late to do it on this blade (and at 3/16" I'm not super worried), but do you guys re-anneal the spine after heat treating for a little flexibility? If so, what's your preferred method? I like the idea of putting clay on the spine for heat treat (spine doesn't drop in temp as quickly as the rest of the blade), but I don't know the right / best method...

There are a few different ways to "Draw the spine" Steve, some will just quench the edge, the clay method which is trickier or here is how I have done it in the past: bring the blade to a polish then harden as the steel requires be it an oil or water hardening alloy.
Bring the blade back to a good polish and then with a torch, lightly play a clean flame along the spine, watching for color closely. You just want a light straw to VERY light hint of violet. Allow to cool. If the blade is such that you are afraid that the heat will get to the edge too quick go to the local welding supply house and get a product called "Heat Stop" paste. Pack the edge with it.
Nothing wrong with used motor oil, it is rich in carbon. The higher polish you have on the blade prior to heat treat the easier the scale cleans off. I have used dark cutting oil with good results.
If you are going for a grained look you just need to kiss it on the belt grinder with the grit of finish you want.

Find out what the critical temperature of the alloy you are using and when you go to the welding supply store pick up "Templaq" stick in the heat range you need, they are a paint stick that melts when you hit the temp you need. Paint line melts, the steel is ready to quench.