and if the STB restarts, its probably doomed.
Not necessarily. STB's are provided with a memory which keeps backup of current running firmware, so in case upgrade is interrupted, STB fetches backup software and can function normally after reboot.
How ever it is recommeded to not disturb upgrade proces by removing signal or power off STB, as it may corrupt bootloader of STB, which may doom the STB as STB will not boot/Power On.