"For example, I've been baking in support for things that the other implementations don't care about (namely i386 support, shadow paging, nested virtualization, support for legacy peripherals, etc) and trying to backfit support for those things into another hypervisor would probably have been just as hard as building it from the ground up."
I don't get it. qemu does all of that already. All he needs to do is implement the same kernel ioctls for OpenBSD that KVM implements on Linux, and he gets all of that for free.