During the weekend I got an old CRT from my parents place and tried out an other rewiring that I found on forums.nekochan.net. Lo and behold, I got an output on my screen.
The black bands on the CRT were not visible with the naked eye (AFAIK this is due to the frequency of the screen refresh and the shutters speed of my phone camera).
This is the latest rewiring:
But not all is well, if I reboot the Octane, the image is gone, replaced by some twirling image. Halting the machine and disconnecting and connecting the power cable of the Octane re-enables good video output. What is causing this behaviour is not yet really clear to me.
I'll try out the CRT later on my O2 and see if it provides a better output than on my DELL monitor.
I'm also waiting for the 'real' SGI 13W3-to-VGA adapters to arrive and try them out. I'll let you know how that turned out.