Maybe starting with the rumpkernel interface is easier and one could already use some of the NetBSD drivers: https://en.wikipedia.org/wiki/Rump_kernel
While I completely agree that NetBSD's Rump kernel would be a great way to go (I've played with it in the past and quite like it!) I do wish there was a similar initiative bound to the Linux kernel.
The closest I've seen is Octavian Purdila's Linux-Kernel-Library project: https://lkml.org/lkml/2015/11/3/706
Not sure how far they've come though. Perhaps Kerla could be a prime mover for them ?