If 9p is the only thing missing in the kernel, then you could use the same trick we’re using with the RHEL derivatives (which also are lacking the module).
For those, we have incus config device add NAME agent disk source=agent:config
This provides what you’d normally get from 9p over a cd-rom drive instead.
That then lets you mount the cdrom (/dev/cdrom or equivalent) and treat it the same as 9p, which typically means copying the content over to /run/incus_agent and then running the agent from there.
If that works in your scenario, that’d save you the rather annoying step of building a custom kernel.