For the benefit of others in the future, yes I have successfully got Cubase 7.07 running on a 2017 iMac that came pre-installed with Mac OS 10.12 Sierra and I’m back in action.
There are some occasional (only seen it twice) minor graphical issues with drawing windows (e.g. I tried to open the Virtual Instruments rack and it indicated that it was opening in the window menu, but it wasn’t drawn on screen), but all audio is performing perfectly. Once I restarted, I haven’t seen the problem again.
So, although it’s not officially supported, 7.07 seems to functioning just fine.