So… A few more details about the cubemap rendering setup:

Cameras set up so that the image-planes meet at the edges, oriented correctly and named correctly.
Now set the Renderer to output a square size and render an image with each cam (if someone finds a way to automate this with a script or something, please let me know – it gets boring after a while…).
Assemble the pieces in Photoshop (or the likes) and done we are: