No exportfs CLI on OS X On Linux, there is exportfs CLI tool which can be used to export NFSs without modifying /etc/exports file. However, I can't find it on OS X 10.8.

Why is it missing? Is there any way to install it or simulate its functionality: dynamically (un)export filesystems?
There are no cli tools for this (not in homebrew, fink or MacPorts). Even the Apple documentation on configuring NFS exports talks about using a text editor to manipulate /etc/exports directly.

There is an older user created shell script for exportfs for Mac OS X. The download link on that page is broken, but the script is available on the archive.org page at mac exportfs. One issue that I noticed is that this script uses the outdated (and unavailable) niutil command for the NetInfo database. You could try using this script as a starting point and modify it to use dscl instead of niutil.