If a -devel version of the port is needed, it should be a mercurial checkout from the source repository for NGINX at the head of the master branch.
This port should be called nginx-mainline as its referred to in the upstream project. Calling this port -devel is misleading, since this is not really a development branch.
The following is a quote from the upstream project's blog:
Which version should I use?
In general, you should deploy the NGINX mainline branch at all times. You may wish to use stable if you are concerned about possible impacts of new features, such as incompatibility with third-party modules or the introduction of bugs in new features.
Some people who are not familiar with the upstream project may shy away from installing a port named -devel when they should not.
Auto-assigned to maintainer osa@FreeBSD.org
Basically, FreeBSD ports tree uses product name for stable version of the port.
For development version - it uses -devel suffix.
Also, please see http://nginx.com/blog/nginx-1-6-1-7-released/ for details.