So, friends, Romans, theoretical female members:
Once I finished up my first lock modification, I started brainstorming more. I've got four I want to make happen in total, so far. This is the second.
OK:
I'd say that it's interesting to note that my final product performs in a dramatically different way than my original concept. This is, specifically, because I did not drill appropriately. I had intended for the existence of the keyblank to keep the check pin in the right place, but when the check pin was extended into the 4th chamber (from the side) it didn't actually interact with the key, as I had drilled too high in the chamber and the cut of the key at that point was rather deep. Please read on to hear what I came up with to salvage this project.
Step 1: Pick out the right lock.
You need a nice thick housing. Most mortice locks I have in my collection work beautifully.
Step 2: Remove most of the pins.
I say most of, because it's pretty important to have the plug line up to the housing EXACTLY* (please see footnote) when you drill through. Thus - I kept the first chamber filled in order to lock the plug into place.
Step 3: Drill!
Ok, it's time to detail the first mistakes I have made in this process.
I used a drill bit. A relatively weak one, and it exploded once it reached the midpoint of the radius of the plug. I was, of course, wearing safety goggles and made it through just fine. However, I then made the additionally stupid mistake of trying to drill with the remaining section of the bit.
EXPLOSION! as you could guess.
So, like a week later, I busted out the dremel and, using a mixture of that slick little cutting ball and a titanium carbide bit, made it the rest of the way through:

and I had drilled too high. This was very frustrating. I picked this lock because it had a mildly eccentric keyway, one I didn't have other blanks for, at the very least, but I had a key that fit it that didn't go to anything else. Anyhow, had I chosen the third chamber to fit, this would have gone fine, but unfortunately in the fourth chamber the check pin only interacts with the 4th key pin, not the blade of the key.
SO!
Step 4: Pinning the checkpin:
I repinned the check pin to hit it's sheerline when the pin in the 4th chamber was present. This isn't as cool as the previous idea, but it still has it's place. Now, if you lift the 4th pin with a pick, the checkpin will shoot into the keyway, preventing you from continuing your picking.
WHICH LED TO ANOTHER PROBLEM!
If you picked the lock, the keypin in the 4th chamber of the lock would remain up, breaching the bible of the lock, and the "driver" of the checkpin would be blocking it's end of the sheer line. Even when I inserted my key, because the blade wasn't actually interacting with the checkpin, the key pin wouldn't descend.
I fixed this like so:

That's me cutting the spring for the checkpin.
So long as the pin stack in 4 had the dominant spring tension/strength, the checkpin would be reset just by virtue of the stack pressing down. By reducing the length of my checkpin spring, I was able to achieve this.
So, now:
Step 5: TAPE!
I used tape to secure the hole I drilled for the checkpin stack.

I'm sorry this wasn't quite so visually stimulating as my last lock modification, but I've got a few other pictures which will follow, below:




Hopefully when I finish up the other two I've got in mind (and maybe reapproach this idea) I'll put it all together in one long PDF.

Enjoy!