Can you explain what is breaking for you? I see one instance of the ant executable being invoked (in the master build file in the "builduserdocs" target). I think that ant is invoked as an executable in order to guarantee the following:
1) enough memory must be given to the JVM to build the docs (the full docs build is a pig)
2) fop.jar needs to be on the classpath in order to build the docs
It may be possible to make the master "release" target fail early if these conditions are not met. Then we may be able to just use the ordinary <ant> task instead of the <exec> task for the docs build.