I noticed those Hugh Craig jacks when I went browsing for info one what is currently available (alas, less and less each year, it seems). Glad to hear they are somewhat less pricey when ordered direct and in quantity - the prices listed on eBay are exorbitant for everything they offer. Jacks at $10 each plus shipping, a pound of music wire for $250, that sort of thing. But it's nice to know they are available, one more option.
For getting rid of slop between jacks and registers, I generally stick a strip or two of self-adhesive label style material on the face of the jack where it bears on the register. A little less work than going the leather on the register route, but only really works for a small gap. I wouldn't want to go more than two strips. I have done it quite a bit, and it seems to hold up over at least several years.
For removing tongue pins, I haven't had to do that too much (haven't run across that many with that design where I actually needed to remove one), but I have tried center pin tool by preference, and it that didn't work, hammered it out with a thin punch. If the jack body is brittle, it doesn't really matter what you do, it will be a risk. You have to use a side of the jack as a bearing surface.
For adding a spring, one option is to drill two holes through the face of the jack. Bend a piece of music wire to the shape and size of spring, with a right angle. Press the non-spring end through one hole, pull taut, bend the end around and press it through the other hole. Possibly make a small retaining bend, and cut the waste length. I think this should work reliably, as long as you make that right angle bend nice and crisp, and the holes are pretty close to the size of the wire. A set of #61-80 wire gauge drill bits is great for this kind of thing.
Regards,
Fred Sturm
"A mind is not a vessel to be filled, but a fire to be kindled." Plutarch