OK, I figured it out, with some help from the other group.
It seems that FreeBSD can mount files, but only export directories,
and can mount subdirectories or files in the exported directory only
with the -alldirs option. And, when using -alldirs, there are restrictions
on what you can export.
But doing it right, the client can mount a single file instead of
a whole directory. This is how diskless clients mount swap.
thanks,
-- glen