Hello Gion,
Thought I would give it a shot, I haven’t tried it myself personally but this looks ok. If you use the Seeed Fusion Gerber CAM file then any information in the milling layer is exported to the outline layer or Gerber mechanical layer. You can confirm that it has been generated correctly by checking the mechanical layer a Gerber viewer.
Another note, it would help if you remove the drill hole as well just so the engineers know which one to follow. Usually, they will follow the milling slots if it is obvious you need it, but it is better to remove any doubt.
Seeed Fusion Eagle CAM file:
<LINK_TEXT text=“http://support.seeedstudio.com/knowledg … from-eagle”>http://support.seeedstudio.com/knowledgebase/articles/1176949-how-to-generate-gerber-and-drill-files-from-eagle</LINK_TEXT>