If your application is a native FreeBSD application, it can not use the v4l-linuxulator stuff. If your application is a linux application, it will use the v4l-linuxulator stuff and can not use the FreeBSD v4l interface directly. All you have to do is to get your application working, you do not have to care about which v4l interface (FreeBSD or Linux) you are using, this will happen automatically.