I would definitely avoid BSDs for Raspberry Pi.
What you are trying to do is not going to work well as that's not how the Raspberry Pis are designed to be used. To do what you are trying to do you will need strong Linux knowledge. I would build a custom Debian system if you are determined to make it work.
You will need the Raspberry Pi kernel and firmware files. Once again, this is not something that's going to be easy to do. I can't really walk you though the process as there is simply not a process to follow.
Could you share your use case?