Dear Jack,
I feel like there is great confusion in this area, so let me clarify this:
The sdcard_spi driver is a driver presenting an SD card as a seekable device while utilizing SPI as a communication channel for the SD card. It contains no balancing algorithm because this is not necessary - data storage SD cards implement this internally, presenting its flash memory as a block device. MFS also does not provide any kind of balancing because it is simply not a task for it - FAT filesystem does not natively support this.
It is certainly possible to implement driver presenting SPI flash as a seekable device, but unless there is some balancing method used it is only safe to use it for read-only filesystem.
Best regards, Pavel