The Arquillian plugin uses mvn exec:java to invoke @Deployment methods within tests that are part of the user's code. We discussed this on the mailinglist. This is a way to invoke code within the project without having to build a classloader that loads the whole user's classpath.
This is how I described it on he mailinglist:

"For the plugin I now just generate a new class with a main method to the project's test-source folder. After that I use an mvn exec:java to run the class. It's less ideal because the plugin now adds a user visible utility class to the project, but it's not really a problem I believe. The exporter class is also runnable outside of Forge, so that's good too."

I'm still polishing the code, but I'll add a link to github when it's checked in.