Victory!! After stepping away from the problem for a couple of months, I decided to try again. I bought a spring-loaded tool called a hammerless center punch with the intention of using it to make a little dimple in the broken-off hook so that I could drill it out without the drill wandering. Well, that little thing produces 3500PSI force, so after making the dimple, I just kept pulling the spring back and letting it fly and the broken piece actually moved backward in the hole until it was sticking a fair way out inside the head. From there, I used a 1/16" hardened centerpunch and hammered at it until it moved the last little bit and finally fell out. Bent the punch, and it took a bit to extricate the punch from the hole (I thought, oh no, this hole is cursed!), but I managed not to enlarge it in the process. The replacement hook tapped in firmly, no problem. I just have to reassemble all the parts I removed and it should finally sew. Thanks for all your input!