Boot screen hangup after upgrade from El Capitan to Sierra
I managed to fix it. For future generations, here's what I did:

Turned out I needed to start the device in safe mode and let Sierra finish its installation - sorting out whatever incompatibility there was automatically. I did try safe mode, but when it showed exactly the same behaviour - a frozen bootup screen with nothing happening - I gave up after 15 minutes and restarted the machine. 

Luckily, I then decided to try safe mode and verbose mode at the same time. To do this, I logged into the recovery console (Cmd + R during bootup), started the terminal, and enabled both modes using

sudo nvram boot-args="-x -v"


(this does not seem to work when booting the machine in Single User mode because of sandbox restrictions; it has to be the terminal in the recovery console)

With this, I managed to see on startup that while the boot screen would be frozen for a long while in safe mode, too, a lot was happening behind the scenes, and eventually I got through to the login screen.

After logging in, the Sierra installer worked for another while (the installation was not yet complete) and rebooted.

(For the record, at this point, I also removed an extension named SteerMouse - but I don't know whether that was what caused the problem. On its website, it does claim to be ready for Sierra.)

Once I was logged in again, I opened a terminal (from within MacOS now, no need for the recovery console) and turned off safe mode and verbose mode again:

sudo nvram boot-args=""


the system is now booted up in normal mode, and everything seems fine.