I haven’t made any headway here yet unfortunately, other things came up. I do know of this project from Google that does something similar though: notebook-kernel-mixer.
I’ve used their Vertex AI Workbenches (skinned jupyterlab) a tiny bit and they seem to use that mixer to enable both local kernels and kernels running in Dataproc (Hadoop) Clusters through Enterprise Gateway. Conceptually, what they’re doing seems to be very similar to the promise of Kernel Provisioners too.
This repo also has some code related to kernel provisioners and gateways but I haven’t had time to dig into it yet: gateway_provisioners