I don't have any idea how to solve your problem, but I'll put my warning to you regarding what you are planning to do like this:
Anytime you install a FreeNAS upgrade your custom DHCP server will be erased.
When FreeNAS 9.1 comes out(hopefully this summer) it will almost certainly break some of the stuff you are doing.
Overall, I don't think what you are doing is very smart. DHCP is supposed to be run on a server that is on and stable, and any FreeNAS upgrade you do has the potential to really backfire. Basically your network is going to go down every time you install an update. Since the USB stick is mounted as read-only that may cause unforeseen issues that won't rear their ugly head until you've spent a lot of time trying to get this to work. Not to mention that the developers don't recommend you install anything on the system drive because it literally has like 9MB free and it needs all that free space for its logs and such. Start messing with it and you could make the entire server unstable.
Just because someone came up with some guide on how to do it doesn't mean you should, or that it will even work well. One youtube video that was linked a few days ago had someone running ZFS on a system with 2GB of RAM. That's completely stupid, but he did it anyway and people are probably mirroring him because "the guide makes it look okay". And some of us senior guys wonder why so many people ignore the manual saying you need 6GB of RAM and wonder why their server ate their data permanently because they had 2GB of RAM.
Good luck!