Success!
The 20,000 pound cylinder was what was needed to do the trick. The 4 ton cheapo Harbor Freight unit didn't budge it, but the 10-ton Enerpac rental popped it free after a disturbing amount of heavy pumping. That key was in there for keeps! Before pulling, I banged on it with an air chisel for a while (straight down, on a sacrificial plate) to possibly vibrate some oil into the gaps, and I gently tapped the key a few times in the three available dimensions to loosen things up. It's been seeped in Kroil for a week or two. I got the 10 ton cylinder in place in the rig, and it came off with a very loud "bang". The keyway looks very clean; it didn't bind anywhere or scar the shaft - it was just age and stubbornness blocking it, apparently. Then, I spun the rig around and used it with some chains to pull off the flywheel, which was also unwilling to just "walk" off the shaft - I needed the hydraulics to pull it off for the full depth of the flywheel/shaft mating surface before it would spin freely.
So here's the list of ingredients to make a gib key puller like mine. The only thing that is not "off the shelf" is the plate that I made to fit over the shaft and key (I've put the PDF of the design one directory level up in the website if you're interested - note that the shaft on the CD is a 2.25" shaft, and not a 2" shaft like on the CS line so it's somewhat bigger than the plate I'll have to make for my 16/2 in order to pull the gib keys there.)
Quan Desc
1 6" Schedule 40 iron pipe, 2.5" diameter
1 10" Schedule 40 iron pipe, 2.5" diameter
1 female-to-female schedule 40 iron pipe adapter
1 Schedule 40 2.5" pipe endcap
1 Schedule 40 2.5" threaded flange, round
1 20,000 pound hydraulic cylinder and pump
2 6' lengths of chain (reasonable weight capacity - maybe 1200#)
4 screw shackles for chain
2 eye bolts, large (1.75" or so for eye diameter)
4 3/4" nut and bolt combinations, about 3" long
4 1" long, 1" diameter steel pipe segments (spacers)
1 1/4" or better steel plate with hole pattern to match pipe flange (for flywheel pull backstop)
1 1/2" steel plate custom-cut with center hole to match shaft and "notch" to fit gib key
I won't go into the description, but you can see from my pictures how it all might go together. This was a big pain in the rear, but in the process I learned how to use a CNC plasma cutter (for the puller plate) so there was an education in there as well. Total cost for everything (including the expensive chain and shackles) was around $150, including the $25 rental of the hydraulic cylinder and pump. Possibly more than might be spent with simpler methods, but I have a permanent solution now for other similar problems, and the same rig can be used for my CS engines if I cut a new puller plate (which I can do for free at the shop.) I suppose someone with a die grinder and some free time could also make the plate themselves without the fancy robot plasma cutter, but I wanted to try something clever at the machine shop instead of doing everything the manual way. :-)
http://www.loligo.com/lister/lister-blackstone-cd/pictures/JT