Okay, I’ve verified that this is a problem with the latest Kernel, but it’s a little beyond my expertise so I’ve raised an issue upstream and will monitor how similar libraries cope with this problem. I made a few attempts to fix it, but didn’t get anywhere.
Update
Joan on the Raspberry Pi forums has suggested a potential fix ( https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=117547&p=806826#p806826 ), and it certainly gets Unicorn HAT up and running for me. I’ll have to regression test it against older versions of the Kernel before releasing it, if anyone’s prepared to try it out you can build from source: https://github.com/pimoroni/unicorn-hat
Clone the Unicorn HAT repo, then cd unicorn-hat/python/rpi-ws281x and sudo ./setup.py install
The Unicorn HAT library itself remains the same, only the rpi-ws281x driver has changed.