Just interjecting a quick thought -- would it be possible to use FAT's reserved sectors for the boot counting? (You can find a description of them under the -R option in the mkfs.vfat man page.) I still think it would be nice to be able to mirror the ESP with mdraid. But if the FAT filesystem is writable by the bootloader, then that won't work. The reserved sectors are specifically reserved for bootloader code by design and if write access by the bootloader/firmware is limited to those sectors then there should be no danger of filesystem corruption if software mirroring is used.