Happily I've just run into exactly the same problem today. Exactly exactly (same boat even). I've started drilling into available end of the bolt but it's taking a long time and it's a real PITA... we've already broken one titanium drill bit. Hopefully not much more to go before I can turn the mast over and grind off the hex cap on the other side.
Couple of questions for you since you've been through it:
1) About how long did it take you to drill out the threaded end of the bolt? How deep did you go?
2) What setup did you use to grind the hex cap off the cap end of the bolt? I'll probably have access to an angle grinder. I don't think I'll have enough clearance to get between the cap and the tang. I'm considering grinding right through the old cast aluminum bracket to essentially cut off the bolt.
3) When the little bits of burning metal are flying around, how did you prevent them from getting all over your gelcoat (and your neighbors' gelcoat)?
4) Any other gotchas during the bracket replacement after that?
I'd say, "Thanks for having this problem three months ago," but I wouldn't wish this work on anybody. Thanks for solving
this problem and posting about it, I guess
p.s. I can confirm that on the C27 the tang bolt hole is wide enough for the bolt, but not for the compression tube. The instructions that come with the spreader bracket retrofit kit from Catalina Direct suggest cutting a notch into a 20-or-so-foot-long PVC pipe and using that to feed the tube into the mast.