It turns out that you can't build a Netbeans Platform application without connecting to the Netbeans IDE first. This has to do with the extra modules, platform components and build harness. Searching the subject online provides a lot of documentation, everything I've found is either out dated, contradictory, or I could only get it working with a new project and not my existing one.
The final solution was so simple, and minimally invasive that I wanted to create this blog post.
There are 4 interesting files for a Netbeans Platform build:
build.xml - Some documentation suggests adding a download-netbeans-rcp target here. That isn't needed. If you have a custom target you can put it here, otherwise there is no need to modify this file.The Netbeans Platform has facilities built into its build script to download all required files. platform.properties is where you need to specify what it should download.
nbproject/build-impl.xml - This is loaded by build.xml, don't modify this file.
nbproject/project.properties - This file seems to identify your modules and the applications name, no need to modify this file either.
nbproject/platform.properties - Modify this file!
# These are used later in the build script
# Provide some URLs
Thats it! It probably took me 5+ hours to find that. Now I can run "ant build-zip" on my Jenkins server and get a zip archive in return.