alt Hacker NewsErlang! Indeed a crazy idea (in a good way!), and while I'm not normally a big fan of unikernels, now you've got me seriously intrigued :)
I've been dabbling in Erlang and OS development myself, my biggest inspirations being Microsoft Singularity and QNX. The former is a C# lookalike of what you're making, or at least that's how it seems from my perspective.
The readme mentions a FreeBSD-like system call interface, but then the drivers and the network stack are written in Erlang, and, as you've mentioned, run in the user land. Is that actually a unikernel design with BEAM running in the kernel, or more of a microkernel hosting BEAM, with it providing device handling and the user space?
The original plan was BEAM on metal, but I had a hard time getting that started... so I pivoted to BEAM from pkg, running on a just enough kernel that exposes only the FreeBSD syscalls that actually get called.
Where that fits in the taxonomy of life, I'm not sure. There is a kernel/userspace boundary (and also a c-code/erlang code boundary in userspace), so it's not quite a unikernel. I wouldn't really call it a microkernel; there's none of the usual microkernel stuff... I just let userspace do i/o ports with the x86 task structure and do memory mapped i/o by letting it mmap anything (more or less). The kernel manages timers/timekeeping and interrupts, Erlang drivers open a socket to get notified when an interrupt fires --- level triggered interrupts would be an issue. Kernel also does thread spawning and mutex support, connects pipes, early/late console output, etc.
If I get through my roadmap (networked demo, uefi/amd64 support, maybe arm64 support, run the otp test suite), I might look again and see if I can eliminate the kernel/userspace divide now that I understand the underneath, but the minimal kernel approach lets me play around with the fun parts, so I'm pretty happy with it. I've got a slightly tweaked dist working and can hotload userspace Erlang code over the network, including the tcp stack, which was the itch I wanted to scratch... nevermind that the tcp stack isn't very good at the moment ;)