A new path: vm86-based Venix emulator
So, I stole the bulk of my old 86sim-based Venix implementation, installed a i386 VM using bhyve on my FreeBSD/amd64 box and write a quick little test program. The test program worked, so in a fit of why not give this a try" I ported the pcvenix.cc from 86sim to being driven from SIGSEGV in vm86 mode. Hello world quickly worked.
I didn't even know what Venix was before coming across this post, but it turns out it was a lightweight UNIX implementation for a variety of platforms.