Решение оказалось неожиданно простым.

Если команда

mount_nfs gate-vasil2:/nfs/distfiles /usr/ports/distfiles

выдавала NFSPROC_NULL: RPC: Timed out,

то

mount_nfs -c gate-vasil2:/nfs/distfiles /usr/ports/distfiles

просто СРАБОТАЛА!

Цитирую man mount_nfs:

     -c      For UDP mount points, do not do a connect(2).  This must be used
             if the server does not reply to requests from the standard NFS
             port number 2049 or replies to requests using a different IP
             address (which can occur if the server is multi-homed).  Setting
             the vfs.nfs.nfs_ip_paranoia sysctl to 0 will make this option the
             default.