I tried just creating a variant.h file (in the appropriate location, down in the same directory as the pins_arduino.h file) that included the relevant definition (#define SDCARD_SPI SPI1) but that didn't do anything. But after a bit of poking around, I found that by adding #include "variant.h" to the relevant pins_arduino.h file the definition was recognised!
This solution also allows me to differentiate between the CubeCell and CubeCell Plus Dev-Boards, which is good because the CubeCell doesn't break out any pins for the second SPI bus. So the variant.h file, which is colocated with the CubeCell Plus pins_arduino.h file and thus is only used when the CubeCell Plus Dev-Board is selected, can be used to specify the default usage of SPI1 for the SD Card Reader, which is pretty much always going to be the desired configuration with the CubeCell Plus Dev-Board.
Not exactly what I was after, but an entirely satisfactory solution. Thanks again all.
You are welcome. I'm glad if I was able to be of assistance. Thanks for taking the time to post an update!
That approach is perfectly valid, but I will mention that there is no magic to the variant.h file specifically. The use of a header file with that name is just a convention the developers of the MKR Zero's "Arduino SAMD Boards (32-bits ARM Cortex-M0+)" platform happened to chose, while the CubeCell platform developers put their core variant declarations in the pins_arduino.h file instead (which is actually the more traditional core variant structure).
If you found it more convenient, you could put the SDCARD_SPI macro definition directly in the pins_arduino.h file instead of adding a variant.h file to the platform, and it would be functionally identical.