I'm still not sure what you're trying to accomplish. As far as I know, you cannot boot over NFS - the NFS client doesn't run at that level. The OS - some OS - has to be up for the client to make the NFS connection. Are you trying to boot via PXE/TFTP or similar, then point the client at an NFS image to use for / or as an install image?
You shouldn't need to do any mapping - you should be able to specify the share explicitly. That being said, there ARE some behind-the-scenes mapping options in /etc/exports. From na_exports(5):
Specifies the actual file system path corresponding to the
exported file system path. You can use this option to move
files to new locations without requiring NFS clients to
mount new file system paths. The actual file system path
you specify must exist. You cannot specify an exported file
system path that consists of a single forward slash (/),
which would mislead some automounters. Note: NFSv4 clients
will not see an exported path using the actual option unless
the export path is only one level deep and is not /vol.