I searched Ebay and found 8mm rivnuts with a tool to install them. It cost just $18 so it was worth the try.
http://stores.ebay.com/HOMEPRO-FASTENERS
If the tool works, then it's worth the purchase. I noticed that the frame is riddled with places to install 8mm rivnuts, so it would be good to have some extras and the tool on hand. I'll post when I get it to report if it works.
In the meantime, I'm going to drill out the damaged rivnut. If the replacement parts and tool don't work, I'm going to order a new rivnut from the Honda parts counter and I'll have them install it.