Wow.

That's going to be a bitch.

I would highly suggest taking it to a mechanic who specializes in Honda engine rebuilds.

There are a couple of ways to tackle the problem, none of which I would ever attempt. I have heard of people using EZ outs and reverse thread drill bits on busted manifold bolts, but never a spark plug. The problem with the EZ outs is the possibility of breaking one off, then you have an even bigger problem.

The other way I've heard of dealing with this is to weld a new nut on top of the busted bolt. In this case, that would require, at a minimum, the head removed.

Good luck, keep us posted, and remember to use anti-seize compound on your new set of plugs.