Hi Spastic Ninja 
Take a look at the files in the Normandy Mod at this link (http://code.google.com/p/zombiesrus5/downloads/list). It might be a good example for you to look at as it has a simplified way of adding capital ships without having to mod the core sins files. This example is for Diplomacy, however the only modification required for Entrenchment is to use the PlayerTech.entity from Entrenchment instead.
Now in regards to the null comment. I'm guessing the file starts with BIN... Assuming this is true you are trying to modify a binary version of the brush file which won't happen. You need to convert the file to text or download the proper file. There are lots of ways to get the proper reference files but the easiest is to download the core set available here, here, or here. You can also download a tool to convert them from here.
- ZombiesRus5
