Yes, I found your kernel module shortly after posting the question last night. It looks good and exactly what I was looking for really - i.e. the underlying work of dealing with the SPI/I2C interfaces is done properly in the kernel and I now only have to look at writing the shims to provide reasonable high level access for C.
The only comment I have is about the RPi ecosystem - compiling a kernel module requires the kernel sources and finding those for the standard Raspbian kernel was not entirely straightforward. I sort of know what I’m doing and it took me about an hour to find everything and get it all sorted - I hate to think how a novice would cope with it!