Hi Shangu,
What I've been doing lately is to set up two render nodes for a given project file. One renderer is for creating the GI cache files, and the second renderer is for rendering the beauty pass which uses the GI cache files. I manually render the first GI cache file, which only takes seconds, so that it's been saved and can be assigned to "read gi cache" settings for the beauty pass render node. Then I submit the project to render using the renderer for the GI cache files. When that's complete, I send off the beauty pass render.
If you have a render controller like Deadline, you can submit projects in this manner, and instruct Deadline not to render the beauty pass until the GI cache render is complete. If you're creating a batch file for command line rendering, you need only to have the command line instructions to render the GI cache render first, and the command line instructions for the beauty pass render on the next line. You can specify which renderer to use for the command line render process.
