EMGO makes a replacement - should cost about US$12 new tops. They have a much larger nut. Apparantly, the reason Honda made it with a small head in the first place was to reduce the likely hood of them getting over-torqued into the engine case.
You might have to grind off the end of the bolt. Then slide off the housing and go to town on the shaft.
Or, you could just take a reciprocating saw and cut right through the housing, filter and bolt
I suppose that's worst-case scenario.
I'm in the exact same boat right now. My bike is in the shop for other reasons, and they're going to try the bolt, but I'm not holding my breath. I did the pipe wrench bit etc. and no luck. I may be taking my dremel tool to the bolt end to see if I can surgically remove the flange so the housing will slide off without damaging the housing itself.