So, I finally got around to trying this. I successfully created an installer media on a USB drive. When I reboot from this device, I end up in a white screen with a crossed out circle sign. I retried the whole process with the Sierra Installer hosted on Mega, same result. I doubt that the drive is broken, because I installed a clean Sierra installation from it just a few days ago, but I don't know how to thoroughly check that, either.
I read up on that problem, but for everyone else who ended up in this, the problem seems to be, that their Mac rebooted without them noticing, trying to boot from an unpatched instatallation. This is not the case with me, I don't get as far as into the installation.
The Hardware I'm trying to get this to run on is a Mac Pro Early 2008, ATI Radeon 5770, Wifi and Bluetooth updated with the kit from osxwifi.com.
Any ideas?