I have assembled a buiildroot patch to create a kmod-nbd package (with the kernel nbd.o that is aready built in the defualt config) and an nbd-client package that includes the nbd-client userspace tool for establishing the connection with an nbd-server. I have found it useful is certain cases and a little lighter weight than nfs + portmap. If anyone is interested I would be happy to post a patch.
Thanks
Robert