For future reference, here is what I ended up doing about the halyard hook on my H16. I post this for others who have this problem later.
I had decided to take the mast cap/sheave off to put SS washers as backing for new rivets and just rivet the halyard hook back in the same holes.
So I took off the sheave, revealing the foam inside the tip of the mast
Probing in the foam to see if I could get some tweezers in there to hold my washers, and guess what? There is a cap under the cap. It's completely sealed off. I cleared out all the foam looking for a way in, and nothing. Apparently all that time I spent caulking the mast cap in years past was pointless.
So at this point I'm pretty frustrated. Capped rivets did not hold before, so I decided to just go back to the first plan: using SS screws. You can see, I decided to put this hook a bit off center, for 2 reasons. First, I was thinking the comptip seam might be weak and contributing to the holes deforming and the rivet releasing. Second, I thought it might save wear and tear on my halyard from raising/lowering the sail - less friction. You can also see that I got it almost straight. Oh well, should hold with some pressure. The drilling was not as hazardous as I feared - the starter holes went in without cracking the comptip.
That foam in the tip only displaces about a cup of water, but I decided what the heck a half pound less to lift on capsize was good. So, I just cut the end off an old pool noodle and stuffed it in there.
I put the cap back on and re-riveted. Now I understand why the Hobie parts list does not call for rivet caps on this part. I think it was just 6-6 SS pan head rivets. BTW, the guys at our sailing center have a great way to pull those SS rivets - they use one of those clamps with a ratcheting pistol grip and pull the riveter handles together with the clamp.
Then I sealed up the holes and all around the new halyard hook with marine silicone. Now I know not to bother with sealing the mast cap anymore.
If there are no further posts here, it will imply this worked. (I'm too disorganized to wait until I know before posting). If it pulls out again or something else happens, I'll try to remember to post back here.