Ah, OK, I finally found the export: it's in the actions on the metabox listing. Unfortunately the resulting .dat-file contains a lot of binary, and cannot be used for easy changes.
Why I wanted to only prototype with the builder, and run it via php and filters:
* I have a few advanced selects where I am creating the options programmatically. I assume this is not possible with the builder interface?
* In my php I can re-use fields easily, and copy&paste between metaboxes. This is not so easy to do with the builder.
* In my php I can also fine tune options faster and see how it looks.
So my ideal is: prototype with builder, export the php, and go from there.