In continuing my experimentation with the PDE Build Ant Tasks, it appears as though you can use those with out actually having to run eclipse. There are a couple of items that you will probably need to do to get them to work:

1. As stated with the Ant4Eclipse post, you'll need to include the appropriate osgi runtime jar.
   
2. Along with that you'll need org.eclipse.core.runtime
3. Also the jar file from org.eclipse.pde.build/lib. You'll find the later in your plugins directory.
4. You'll want to create a taskDef file that has the necessary tasks defined. These are items like eclipse.generateFeature, eclipse.buildScript. The task defs should point to the appropriate classes. i.e. org.eclipse.pde.internal.build.tasks.BuildScriptGeneratorTask for the eclipse.buildScript task.

Theoretically, since eclipse bundles are just jar files, they can be used outside of eclipse. With the appropriate setup, you can then run a build with out having to actually launch eclipse to launch ant. You can have Ant use the build scripts with out needing eclipse.

I still need to test this, and some additional jars may be necessary, but from what I can tell at least 90% of the tasks don't rely specifically on eclipse resources, except for the CoreRuntime exception class.