Okay, I figured that I must be missing something here, so I revisited the situation. I took it all apart to see what could be affecting the bolt.
The bolt by itself has plenty of length through the Kaoko. I reinstalled it, without the washer for the handguard, and Voila! It seats:
As you can see, the washer has significant depth to it:
Normally, I'd see about going without it (even though it's not aesthetically pleasing or symmetrical). However, the bolt hole in the handguard is just a fraction smaller than the bolt head. So the washer is actually important:
Someone, somewhere has a 90mm bolt, I'm sure. More to follow tomorrow.