Though skeptical, I took Irondog's warning about my homemade tool deforming under pressure seriously. I found that a length of 1/2" copper pipe was almost exactly the same OD as the ID of the clutch hole/gallery. Wrapping the bolt with a couple layers of electrical tape gave the pipe a tight fit.
I also decided to tap the end of the bolt so I could thread a long machine screw in for removal in case it didn't work. The 18mm x 1.5 pitch bolt cost me a little over $3. It worked like a charm.
Here's a picture: