Assuming that you have a working FreeBSD DHCP/NFS server on 192.168.1.10 already configured and that your network is 192.168.1.0/255.255.255.0

I also assume that you can access the serial console of the machine to be netbooted from the host via tip(1), conserver-com, etc.

host testbox {
     fixed-address 192.168.1.101;
     hardware ethernet 00:16:3e:00:00:04;
     next-server 192.168.1.10;
     filename "pxeboot.9";
     option root-path "192.168.1.10:/pxeroot/9.0-RELEASE/";
}
tftp    dgram   udp     wait    root    /usr/libexec/tftpd      tftpd -l -s /tftpboot
ttyu0   "/usr/libexec/getty std.9600"   vt100   on secure
#/dev/iso9660/FREEBSD_INSTALL / cd9660 ro 0 0
192.168.1.10:/pxeroot/9.0-RELEASE/ / nfs ro 0 0
host testbox {
     fixed-address 192.168.1.101;
     hardware ethernet 00:16:3e:00:00:04;
     #next-server 192.168.1.10;
     #filename "pxeboot.9";
     #option root-path "192.168.1.10:/pxeroot/9.0-RELEASE/";
}
ttyu0   "/usr/libexec/getty std.9600"   vt100   on secure