This was possible before the introduction of userspace nfs, but not any more. Userspace nfs has become the only option available for qemu nfsroot, the unfs server is always started and nfsprog/mountprog cannot be changed. Kernel nfs may still be needed occasionally, for: 1. it doesn't require pseudo thus faster, 2. it can be mounted by normal clients (by using standard prog number)
Using a kernel NFS server is still possible, but you have to know what you're doing and not make use of the poky-qemu script. When I first took on the task of adding userspace NFS support, the existing poky NFS script was no longer maintained and had problems; Richard asked me to remove it. We can add this ability back but this should be considered a feature addition for after 1.0.
The has been no demand for this over the years, closing. Someone can add it if wanted.