Ok, so the two piece hook didn't actually survive my weight for long. The simpler flat hook did however, and it seemed like it could do so indefinitely. I weigh about 200 lbs. and was swinging around a bit so a couple extra lbs. of centrifugal force too. Tied to 3mm Kevlar line. Don't actually use this for anything life threatening, obviously.
Printed with PLA 100% infill, a raft is a must because it warps so much while printing. The two piece file is included if anyone wants to improve on it, it broke at the slot so a larger hook with more material around that area may help, but the goal was to make a weight bearing grappling hook that was as small as possible.
The slots are 1/16" wider than actual dimensions to allow for shrinkage which was almost a perfect press fit for me, obviously this may vary wildly depending on your filament and machine.