resource MyResource {
    action list
    action inputData
    action test
    action generateData
    action sendTo
    ...
}

public class MyDslFileSystemAccess extends EclipseResourceFileSystemAccess {

	protected static final String ONCE_FOLDER_NAME = "src-once";
	
	// Just a quick hack - need access to the workspace root in here, and it's private in EclipseResourceFileSystemAccess
	@Inject
	private IWorkspaceRoot rootThis;
	
	public void setRootThis(IWorkspaceRoot root) {
		this.rootThis = root;
	}
	
	@Override
	public void deleteFile(String fileName) {
		// Only delete the file if it's not in src-once
		if (!fileName.contains(ONCE_FOLDER_NAME)) {
			super.deleteFile(fileName);
		}
	}

	@Override
	public void generateFile(String fileName, String slot, CharSequence contents) {
		if (shouldGenerate(fileName, slot)) {
			super.generateFile(fileName, slot, contents);
			
			// Some redundancy here: This is a repeat of what super does to determine the file
			// Needs to be done to set the file back to "not derived" so that it can be edited
			String outletPath = getPathes().get(slot);
			IFile file = rootThis.getFile(new Path(outletPath + "/" + fileName));
			if (slot.equals(ONCE_FOLDER_NAME)) {
				try {
					file.setDerived(false);
				} catch (CoreException e) {
					e.printStackTrace();
				}
			}
		}
		
	}
	
	private boolean shouldGenerate(String fileName, String slot) {
		if (!slot.equals(ONCE_FOLDER_NAME)) {
			return true;
		}
		String outletPath = getPathes().get(slot);
		IFile file = rootThis.getFile(new Path(outletPath + "/" + fileName));
		if (!file.exists()) {
			return true;
		}
		return false;
		
	}
	
}

public class MyDslBuilderParticipant extends JavaProjectBasedBuilderParticipant {

	@Override
	protected IFileSystemAccess getConfiguredFileSystemAccess(IFolder srcGenFolder, IAcceptor<String> newFileAcceptor) {
		MyDslFileSystemAccess access = new MyDslFileSystemAccess();
		access.setOutputPath(srcGenFolder.getFullPath().toString());
		access.setOutputPath(
				MyDslFileSystemAccess.ONCE_FOLDER_NAME,
				srcGenFolder.getProject().getFolder(MyDslFileSystemAccess.ONCE_FOLDER_NAME)
						.getFullPath().toString());
		access.setNewFileAcceptor(newFileAcceptor);
		access.setRoot(srcGenFolder.getWorkspace().getRoot());
		access.setRootThis(srcGenFolder.getWorkspace().getRoot());
		return access; 
	}
	
}

// generate to default (src-gen)
fsa.generateFile(
  "Interface.java",
  g.compileInterface)
// Generate to src-once
  fsa.generateFile(
  "ClassImpl.java",
  "src-once",
  g.compileImpl)

public Class<? extends org.eclipse.xtext.builder.IXtextBuilderParticipant> bindIXtextBuilderParticipant() {
		return MyDslBuilderParticipant.class;
	}