I would do as you said - stamp the ricasso pre-HT and finish the blade as normal after HT. Stamp deeply, as you will be sanding/grinding away some of te surface in post HT work.
If the steel is annealed, there should be no risk for stamping a single letter.
Making (or buying) a stamping holder is a wise choice. It is a stand with a guide/holder that is positioned above a heavy base plate on the bottom. The blade is set on the base plate and positioned under the stamp which is in the guide. Once all is in place right and you are holding the blade firmly to prevent "bounce" strike the stamp ONE TIME with a 2# maul. A little practice on some scrap and you will find the correct amount of force to use.
Other folks convert a 1-ton or 2-ton arbor press into a stamping press. This works fine for knives once you practice a bit. A two-ton or bigger press may be able to make the impression buy itself, but most folks use a hammer to apply the extra jolt of power.
Some better arbor presses come with the end having a hole and set screw for the tooling or lettering. If that is what you get, you may need to cut off your stamps and grind the ends to fit.
I made one from a cheap HF press with just a round shaft. I drilled a 1" deep hole in the end of the arbor shaft to fit my 1/4" letter/number stamps (.355" diameter hole). I drilled and tapped a set screw for the hole. Stick the stamp in the hole, apply some down pressure on a piece of scrap with the pressure lever, and tighten the set screw to hold it there. (You want the top of the stamp bottomed out solid in the hole).
Pull down on the ram to apply strong pressure on the stamp against the blade and give the top of the arbor a whack with a hammer. It does not take as much hammer force as using a stamping jig above, as the press is already applying a good amount of force. Again, experiment to get the right combination.